YSK: If your instance updated to 0.18.1, there's new themes in your profile settings, including compact and pure black themes! (for the website version of lemmy, version number at bottom of page)
Why YSK: Because that's friggin awesome?!
Screenshots (themes also have dark versions):
Default:
Default-compact:
"Vaporwave":
There's also an odd "i386" theme, along with a "pure black" darkly.
The settings don't seem to stick for me for some reason. I hit save and it looks good on my profile page but then when I navigate back to lemmy.world it goes back to the "browser default" setting.