i made a blender theme i call "blackout" (i made this in 2.79, so i don't know if it will work in 2.91 but download link will be comming hopefully soon) :
Download – Studio LLB
Blender 2.79a - Neowin
Blender 2.7 Download (Free) - blender.exe
Blender 2.79 Addon Size Me - YouTube
2.79 — blender.org
Blender 2.79b Bugfix Release Now Available - BlenderNation