Material Theme for Visual Studio 2019
Check out the colour themes from Dainty for Visual Studio.
https://dainty.site/vs
You will need Node and Yarn installed to build the theme(s) you want. Not all the themes work perfectly with VS2019 but they do have a couple of Material Themes.
You can install the Color Themes for Visual Studio extension, from Microsoft DevLabs.
As of the date of this post, the extension includes a Material Dark theme, among others (refer to the GiHub Repository for more information).
NOTE: You can also customise an existing theme, using the Color Theme Editor, for VS 2017 and VS 2019.