WPF TabItem Header Styling

Try this style instead, it modifies the template itself. In there you can change everything you need to transparent:

<Style TargetType="{x:Type TabItem}">
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="{x:Type TabItem}">
        <Grid>
          <Border Name="Border" Margin="0,0,0,0" Background="Transparent"
                  BorderBrush="Black" BorderThickness="1,1,1,1" CornerRadius="5">
            <ContentPresenter x:Name="ContentSite" VerticalAlignment="Center"
                              HorizontalAlignment="Center"
                              ContentSource="Header" Margin="12,2,12,2"
                              RecognizesAccessKey="True">
              <ContentPresenter.LayoutTransform>
            <RotateTransform Angle="270" />
          </ContentPresenter.LayoutTransform>
        </ContentPresenter>
          </Border>
        </Grid>
        <ControlTemplate.Triggers>
          <Trigger Property="IsSelected" Value="True">
            <Setter Property="Panel.ZIndex" Value="100" />
            <Setter TargetName="Border" Property="Background" Value="Red" />
            <Setter TargetName="Border" Property="BorderThickness" Value="1,1,1,0" />
          </Trigger>
          <Trigger Property="IsEnabled" Value="False">
            <Setter TargetName="Border" Property="Background" Value="DarkRed" />
            <Setter TargetName="Border" Property="BorderBrush" Value="Black" />
            <Setter Property="Foreground" Value="DarkGray" />
          </Trigger>
        </ControlTemplate.Triggers>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>

While searching for a way to round tabs, I found Carlo's answer and it did help but I needed a bit more. Here is what I put together, based on his work. This was done with MS Visual Studio 2015.

The Code:

<Window x:Class="MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:MealNinja"
        mc:Ignorable="d"
        Title="Rounded Tabs Example" Height="550" Width="700" WindowStartupLocation="CenterScreen" FontFamily="DokChampa" FontSize="13.333" ResizeMode="CanMinimize" BorderThickness="0">
    <Window.Effect>
        <DropShadowEffect Opacity="0.5"/>
    </Window.Effect>
    <Grid Background="#FF423C3C">
        <TabControl x:Name="tabControl" TabStripPlacement="Left" Margin="6,10,10,10" BorderThickness="3">
            <TabControl.Resources>
                <Style TargetType="{x:Type TabItem}">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="{x:Type TabItem}">
                                <Grid>
                                    <Border Name="Border" Background="#FF6E6C67" Margin="2,2,-8,0" BorderBrush="Black" BorderThickness="1,1,1,1" CornerRadius="10">
                                        <ContentPresenter x:Name="ContentSite" ContentSource="Header" VerticalAlignment="Center" HorizontalAlignment="Center" Margin="2,2,12,2" RecognizesAccessKey="True"/>
                                    </Border>
                                    <Rectangle Height="100" Width="10" Margin="0,0,-10,0" Stroke="Black" VerticalAlignment="Bottom" HorizontalAlignment="Right" StrokeThickness="0" Fill="#FFD4D0C8"/>
                                </Grid>
                                <ControlTemplate.Triggers>
                                    <Trigger Property="IsSelected" Value="True">
                                        <Setter Property="FontWeight" Value="Bold" />
                                        <Setter TargetName="ContentSite" Property="Width" Value="30" />
                                        <Setter TargetName="Border" Property="Background" Value="#FFD4D0C8" />
                                    </Trigger>
                                    <Trigger Property="IsEnabled" Value="False">
                                        <Setter TargetName="Border" Property="Background" Value="#FF6E6C67" />
                                    </Trigger>
                                    <Trigger Property="IsMouseOver" Value="true">
                                        <Setter Property="FontWeight" Value="Bold" />
                                    </Trigger>
                                </ControlTemplate.Triggers>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                    <Setter Property="HeaderTemplate">
                        <Setter.Value>
                            <DataTemplate>
                                <ContentPresenter Content="{TemplateBinding Content}">
                                    <ContentPresenter.LayoutTransform>
                                        <RotateTransform Angle="270" />
                                    </ContentPresenter.LayoutTransform>
                                </ContentPresenter>
                            </DataTemplate>
                        </Setter.Value>
                    </Setter>
                    <Setter Property="Background" Value="#FF6E6C67" />
                    <Setter Property="Height" Value="90" />
                    <Setter Property="Margin" Value="0" />
                    <Setter Property="Padding" Value="0" />
                    <Setter Property="FontFamily" Value="DokChampa" />
                    <Setter Property="FontSize" Value="16" />
                    <Setter Property="VerticalAlignment" Value="Top" />
                    <Setter Property="HorizontalAlignment" Value="Right" />
                    <Setter Property="UseLayoutRounding" Value="False" />
                </Style>
                <Style x:Key="tabGrids">
                    <Setter Property="Grid.Background" Value="#FFE5E5E5" />
                    <Setter Property="Grid.Margin" Value="6,10,10,10" />
                </Style>
            </TabControl.Resources>
            <TabItem Header="Planner">
                <Grid Style="{StaticResource tabGrids}"/>
            </TabItem>
            <TabItem Header="Section 2">
                <Grid Style="{StaticResource tabGrids}"/>
            </TabItem>
            <TabItem Header="Section III">
                <Grid Style="{StaticResource tabGrids}"/>
            </TabItem>
            <TabItem Header="Section 04">
                <Grid Style="{StaticResource tabGrids}"/>
            </TabItem>
            <TabItem Header="Tools">
                <Grid Style="{StaticResource tabGrids}"/>
            </TabItem>
        </TabControl>
    </Grid>
</Window>

Screenshot: