I made a dark theme for NinSheetMusic. It's almost definitely not perfect, and I probably won't be updating it much. But it works fairly well, both for the forum and the main site.
Just get the Stylus browser extension, then click the Install button on this page (https://userstyles.world/style/13518/dark-ninsheetmusic).
This is probably not the best place to post it, but I really don't know what else to do.
ooooh that's cool!