How to customize the background color of the selected item?

Hello,

Our theme generator creates hundreds of brush resources that are used throughout our themes.  The color used for highlights is typically driven by the ThemeDefinition.PrimaryAccentColorFamilyName property, which defaults to Blue.  You can use our Theme Designer application to alter high level theme definition properties.  

If you want to start with our MetroLight or MetroDark themes as a base, then you'd start with a MetroThemeDefinition instance with a custom theme name, and use ThemeIntent.Light or ThemeIntent.Dark.  Then alter other properties on the theme definition and use it to generate the theme.  The Theme Designer's Presets tab lets you select Metro Light and Metro Dark to use as a base.  Alter properties as desired and go to the Code tab to see the code that you should insert into your app startup.

The Theme Designer's Resource Browser tab lets you see the resources that are generated.  If you filter the list by "ListItem" you can see all the brushes that TreeListViewItem (and other lists) will use.  This documentation topic tells you how to customize brushes if you need further customization beyond what our theme definition's properties will generate.