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. Get it at https://tampermonkey.net/. Install userscript from here: https://github.com/infinitytec/3.Darker/raw/master/3darker.user.js
It is highly reccommended that you also use 2.0 Colors, as it makes the blocks be less bright. Get it at https://github.com/infinitytec/2.0-Colors
Note that this falls under the MIT License, which I found appropriate.