Use Header colors for desktop style ToolBar

This will fall back to the current Window colors when using a color scheme without Header colors, so there is no visual change right now. However this ensures that we are prepared for the new Breeze color scheme once it lands.

Here's how it will look once the color scheme changes from plasma/breeze!2 (merged) are merged:

(The search field lives in a ToolBar)

cc @teams/vdg