first, please always refer to the version of librecad you use.
in 2.2.2 (alpha branch, located there
https://github.com/LibreCAD/LibreCAD/releases/tag/2.2.2_alpha-latest)
1) custom colored icons are supported, the overall support of dark theme - most probably will be added later. Demo
https://youtu.be/mbYkwPAdKJU2) well, that's interesting request, yet here we're also limited by underling drawing format - DXF. I'll think what may be achieved there.
3) Check the Layer Tree widget in 2.2.1 and 2.2.2 - a bit outdated demo is there -
https://youtu.be/kFLRBU7FSCA