WiX bootstrapper theme file?

(This is an updated version of @BryanJ's answer - thanks Bryan)

If you need the latest WiX theme file download the "wixXXX-debug.zip" file from here:
https://wixtoolset.org/releases/
...which contains the theme file \src\ext\BalExtension\wixstdba\Resources\RtfTheme.xml

The WiX 3.10 RtfTheme.xml file is this:

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->


<Theme xmlns="http://wixtoolset.org/schemas/thmutil/2010">
    <Window Width="485" Height="300" HexStyle="100a0000" FontId="0">#(loc.Caption)</Window>
    <Font Id="0" Height="-12" Weight="500" Foreground="000000" Background="FFFFFF">Segoe UI</Font>
    <Font Id="1" Height="-24" Weight="500" Foreground="000000">Segoe UI</Font>
    <Font Id="2" Height="-22" Weight="500" Foreground="666666">Segoe UI</Font>
    <Font Id="3" Height="-12" Weight="500" Foreground="000000" Background="FFFFFF">Segoe UI</Font>
    <Font Id="4" Height="-12" Weight="500" Foreground="ff0000" Background="FFFFFF" Underline="yes">Segoe UI</Font>

    <Image X="11" Y="11" Width="64" Height="64" ImageFile="logo.png" Visible="yes"/>
    <Text X="80" Y="11" Width="-11" Height="64" FontId="1" Visible="yes" DisablePrefix="yes">#(loc.Title)</Text>

    <Page Name="Help">
        <Text X="11" Y="80" Width="-11" Height="30" FontId="2" DisablePrefix="yes">#(loc.HelpHeader)</Text>
        <Text X="11" Y="112" Width="-11" Height="-35" FontId="3" DisablePrefix="yes">#(loc.HelpText)</Text>
        <Button Name="HelpCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.HelpCloseButton)</Button>
    </Page>
    <Page Name="Install">
        <Richedit Name="EulaRichedit" X="11" Y="80" Width="-11" Height="-70" TabStop="yes" FontId="0" HexStyle="0x800000" />
        <Checkbox Name="EulaAcceptCheckbox" X="-11" Y="-41" Width="260" Height="17" TabStop="yes" FontId="3" HideWhenDisabled="yes">#(loc.InstallAcceptCheckbox)</Checkbox>
        <Button Name="OptionsButton" X="-171" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.InstallOptionsButton)</Button>
        <Button Name="InstallButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.InstallInstallButton)</Button>
        <Button Name="WelcomeCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.InstallCloseButton)</Button>
    </Page>
    <Page Name="Options">
        <Text X="11" Y="80" Width="-11" Height="30" FontId="2" DisablePrefix="yes">#(loc.OptionsHeader)</Text>
        <Text X="11" Y="121" Width="-11" Height="17" FontId="3" DisablePrefix="yes">#(loc.OptionsLocationLabel)</Text>
        <Editbox Name="FolderEditbox" X="11" Y="143" Width="-91" Height="21" TabStop="yes" FontId="3" FileSystemAutoComplete="yes" />
        <Button Name="BrowseButton" X="-11" Y="142" Width="75" Height="23" TabStop="yes" FontId="3">#(loc.OptionsBrowseButton)</Button>
        <Button Name="OptionsOkButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.OptionsOkButton)</Button>
        <Button Name="OptionsCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.OptionsCancelButton)</Button>
    </Page>
    <Page Name="FilesInUse">
      <Text X="11" Y="80" Width="-11" Height="30" FontId="2" DisablePrefix="yes">#(loc.FilesInUseHeader)</Text>
      <Text X="11" Y="121" Width="-11" Height="34" FontId="3" DisablePrefix="yes">#(loc.FilesInUseLabel)</Text>
      <Text Name="FilesInUseText" X="11" Y="150" Width="-11" Height="-86" FontId="3" DisablePrefix="yes" HexStyle="0x0000C000"></Text>

      <Button Name="FilesInUseCloseRadioButton" X="11" Y="-60" Width="-11" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes" HexStyle="0x000009">#(loc.FilesInUseCloseRadioButton)</Button>
      <Button Name="FilesInUseDontCloseRadioButton" X="11" Y="-40" Width="-11" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes" HexStyle="0x000009">#(loc.FilesInUseDontCloseRadioButton)</Button>

      <Button Name="FilesInUseOkButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.FilesInUseOkButton)</Button>
      <Button Name="FilesInUseCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.FilesInUseCancelButton)</Button>
    </Page>
    <Page Name="Progress">
        <Text X="11" Y="80" Width="-11" Height="30" FontId="2" DisablePrefix="yes">#(loc.ProgressHeader)</Text>
        <Text X="11" Y="121" Width="70" Height="17" FontId="3" DisablePrefix="yes">#(loc.ProgressLabel)</Text>
        <Text Name="OverallProgressPackageText" X="85" Y="121" Width="-11" Height="17" FontId="3" DisablePrefix="yes">#(loc.OverallProgressPackageText)</Text>
        <Progressbar Name="OverallCalculatedProgressbar" X="11" Y="143" Width="-11" Height="15" />
        <Button Name="ProgressCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.ProgressCancelButton)</Button>
    </Page>
    <Page Name="Modify">
        <Text X="11" Y="80" Width="-11" Height="30" FontId="2" DisablePrefix="yes">#(loc.ModifyHeader)</Text>
        <Button Name="RepairButton" X="-171" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.ModifyRepairButton)</Button>
        <Button Name="UninstallButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.ModifyUninstallButton)</Button>
        <Button Name="ModifyCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.ModifyCloseButton)</Button>
    </Page>
    <Page Name="Success">
        <Text Name="SuccessHeader" X="11" Y="80"  Width="-11" Height="30" FontId="2" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.SuccessHeader)</Text>
        <Text Name="SuccessInstallHeader" X="11" Y="80" Width="-11" Height="30" FontId="2" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.SuccessInstallHeader)</Text>
        <Text Name="SuccessRepairHeader" X="11" Y="80" Width="-11" Height="30" FontId="2" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.SuccessRepairHeader)</Text>  
        <Text Name="SuccessUninstallHeader" X="11" Y="80" Width="-11" Height="30" FontId="2" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.SuccessUninstallHeader)</Text>        
        <Button Name="LaunchButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.SuccessLaunchButton)</Button>
        <Text Name="SuccessRestartText" X="-11" Y="-51" Width="400" Height="34" FontId="3" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.SuccessRestartText)</Text>
        <Button Name="SuccessRestartButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.SuccessRestartButton)</Button>
        <Button Name="SuccessCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.SuccessCloseButton)</Button>
    </Page>
    <Page Name="Failure">
        <Text Name="FailureHeader" X="11" Y="80" Width="-11" Height="30" FontId="2" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.FailureHeader)</Text>
        <Text Name="FailureInstallHeader" X="11" Y="80" Width="-11" Height="30" FontId="2" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.FailureInstallHeader)</Text>
        <Text Name="FailureUninstallHeader" X="11" Y="80" Width="-11" Height="30" FontId="2" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.FailureUninstallHeader)</Text>
        <Text Name="FailureRepairHeader" X="11" Y="80" Width="-11" Height="30" FontId="2" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.FailureRepairHeader)</Text>      
        <Hypertext Name="FailureLogFileLink" X="11" Y="121" Width="-11" Height="42" FontId="3" TabStop="yes" HideWhenDisabled="yes">#(loc.FailureHyperlinkLogText)</Hypertext>
        <Hypertext Name="FailureMessageText" X="22" Y="163" Width="-11" Height="51" FontId="3" TabStop="yes" HideWhenDisabled="yes" />
        <Text Name="FailureRestartText" X="-11" Y="-51" Width="400" Height="34" FontId="3" HideWhenDisabled="yes" DisablePrefix="yes">#(loc.FailureRestartText)</Text>
        <Button Name="FailureRestartButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.FailureRestartButton)</Button>
        <Button Name="FailureCloseButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.FailureCloseButton)</Button>
    </Page>
</Theme>

If you download the WiX source, those theme files are located in src\ext\BalExtension\wixstdba\Resources.

The RtfTheme is an xml file:

<?xml version="1.0" encoding="utf-8"?>
<Theme xmlns="http://wixtoolset.org/schemas/thmutil/2010">
    <Window Width="485" Height="300" HexStyle="100a0000" FontId="0">#(loc.Caption)</Window>
    <Font Id="0" Height="-12" Weight="500" Foreground="000000" Background="FFFFFF">Segoe UI</Font>
    <Font Id="1" Height="-24" Weight="500" Foreground="000000">Segoe UI</Font>
    <Font Id="2" Height="-22" Weight="500" Foreground="666666">Segoe UI</Font>
    <Font Id="3" Height="-12" Weight="500" Foreground="000000" Background="FFFFFF">Segoe UI</Font>
    <Font Id="4" Height="-12" Weight="500" Foreground="ff0000" Background="FFFFFF" Underline="yes">Segoe UI</Font>

    <Image X="11" Y="11" Width="64" Height="64" ImageFile="logo.png" Visible="yes"/>
    <Text X="80" Y="11" Width="-11" Height="64" FontId="1" Visible="yes">#(loc.Title)</Text>

    <Page Name="Install">
        <Richedit Name="EulaRichedit" X="11" Y="80" Width="-11" Height="-70" TabStop="yes" FontId="0" HexStyle="0x800000" />
        <Checkbox Name="EulaAcceptCheckbox" X="-11" Y="-41" Width="246" Height="17" TabStop="yes" FontId="3" HideWhenDisabled="yes">#(loc.InstallAcceptCheckbox)</Checkbox>
        <Button Name="OptionsButton" X="-171" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.InstallOptionsButton)</Button>
        <Button Name="InstallButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.InstallInstallButton)</Button>
        <Button Name="WelcomeCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.InstallCloseButton)</Button>
    </Page>
    <Page Name="Options">
        <Text X="11" Y="80" Width="-11" Height="30" FontId="2">#(loc.OptionsHeader)</Text>
        <Text X="11" Y="121" Width="-11" Height="17" FontId="3">#(loc.OptionsLocationLabel)</Text>
        <Editbox Name="FolderEditbox" X="11" Y="143" Width="-91" Height="21" TabStop="yes" FontId="3" FileSystemAutoComplete="yes" />
        <Button Name="BrowseButton" X="-11" Y="142" Width="75" Height="23" TabStop="yes" FontId="3">#(loc.OptionsBrowseButton)</Button>
        <Button Name="OptionsOkButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.OptionsOkButton)</Button>
        <Button Name="OptionsCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.OptionsCancelButton)</Button>
    </Page>
    <Page Name="Progress">
        <Text X="11" Y="80" Width="-11" Height="30" FontId="2">#(loc.ProgressHeader)</Text>
        <Text X="11" Y="121" Width="70" Height="17" FontId="3">#(loc.ProgressLabel)</Text>
        <Text Name="OverallProgressPackageText" X="85" Y="121" Width="-11" Height="17" FontId="3">#(loc.OverallProgressPackageText)</Text>
        <Progressbar Name="OverallCalculatedProgressbar" X="11" Y="143" Width="-11" Height="15" />
        <Button Name="ProgressCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.ProgressCancelButton)</Button>
    </Page>
    <Page Name="Modify">
        <Text X="11" Y="80" Width="-11" Height="30" FontId="2">#(loc.ModifyHeader)</Text>
        <Button Name="RepairButton" X="-171" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.ModifyRepairButton)</Button>
        <Button Name="UninstallButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.ModifyUninstallButton)</Button>
        <Button Name="ModifyCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.ModifyCloseButton)</Button>
    </Page>
    <Page Name="Success">
        <Text X="11" Y="80" Width="-11" Height="30" FontId="2">#(loc.SuccessHeader)</Text>
        <Button Name="LaunchButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.SuccessLaunchButton)</Button>
        <Text Name="SuccessRestartText" X="-11" Y="-51" Width="400" Height="34" FontId="3" HideWhenDisabled="yes">#(loc.SuccessRestartText)</Text>
        <Button Name="SuccessRestartButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.SuccessRestartButton)</Button>
        <Button Name="SuccessCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.SuccessCloseButton)</Button>
    </Page>
    <Page Name="Failure">
        <Text X="11" Y="80" Width="-11" Height="30" FontId="2">#(loc.FailureHeader)</Text>
        <Hypertext Name="FailureLogFileLink" X="11" Y="121" Width="-11" Height="42" FontId="3" TabStop="yes" HideWhenDisabled="yes">#(loc.FailureHyperlinkLogText)</Hypertext>
        <Hypertext Name="FailureMessageText" X="22" Y="163" Width="-11" Height="51" FontId="3" TabStop="yes" HideWhenDisabled="yes" />
        <Text Name="FailureRestartText" X="-11" Y="-51" Width="400" Height="34" FontId="3" HideWhenDisabled="yes">#(loc.FailureRestartText)</Text>
        <Button Name="FailureRestartButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0" HideWhenDisabled="yes">#(loc.FailureRestartButton)</Button>
        <Button Name="FailureCloseButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.FailureCloseButton)</Button>
    </Page>
</Theme>