A UserCSS compatible userstyle manager is required, those are recommended:

Stylus for Firefox, Chrome or Opera.
Cascadea for Safari.

Mobile:
Stylus for Firefox Nighly (using this)

Note: Activate the websites own dark theme if they have one, as I (re)write over that.

Click here to install Last.fm stylesheet.

Click here to install Discogs stylesheet.

Google (discontinued) You can get dark mode in settings at the bottom of google.com