Scratch 3.0 Darker Theme Userscript using my general theme framework, which can be found at https://github.com/infinitytec/3.Dark.

I use the Tampermonkey extension for userscripts. Install userscript from here: https://github.com/infinitytec/3.Darker/raw/master/3darker.user.js

Note that this falls under the MIT License, which I found appropriate.