This diff allows to make custom colors in face orientation overlay.
It solves 2 problems.
- Users asks that in 2.8x in the viewport the colour difference between the frontface and backface appears to have entirely disappeared https://developer.blender.org/T59309. Using custom face orientation colors, with transparent frontface and semi-transparent backface, overlay could be left on all the time, so it'll just be his normal way of working. If a face normal accidentally gets flipped, or a hole accidentally gets cut in a mesh, it'll immediately show up in red (black or any color).
In current situation toggling blue/red mode is not a best way for working.
- Second is ability to theming face orientation overlay. It can be nice to get eye-friendly colors. Some examples:
Suggest on rightclickselect: https://blender.community/c/rightclickselect/RVcbbc/