We do support that kind of thing via our use of DynamicImage and ImageProvider as described in the documentation. Docking windows do use DynamicImage in their templates but after looking, they don't currently track the text Foreground, so monochrome vector image adaptation will only work on a theme basis in the 22.1 version and not as the Foreground color changes for states.
We have updated it so that monochrome vector image adaptation will work in v23.1 once that is released. You will be able to use monochrome vector images like this and have them track the current text Foreground properly in that version:
<GeometryDrawing Brush="#30000000" Geometry="M 0,16 A 16,16 0 1 1 32,16 A 16,16 0 1 1 0,16 Z" />
<GeometryDrawing Brush="#000000" Geometry="M 5.75,17.25 L 8.75,14.25 L 13,19 L 23.5,8.25 L 26.25,11.25 L 13,24.5 Z" />
Note that this particular DrawingImage is using an ImageProvider example in the Dynamic Image Intro QuickStart.