We all know how windows themeing is hard and you need to patch and modify system files.

but for web-based apps its a completely different approach, we can use an extension called stylus

There are many styles and themes we can find for pretty much all popular apps, for example there's github moonlight as a theme for github which you have many customization options.

Almost all themes are on userstyles.org where you will find most themes, but I like to use userstyles archive because userstyles.org takes ages to load.

But if you want to go a different approach you can make your own theme. Its really simple to make it if you know css. Just inspect element the elements you want themed and with a bit of css and putting the code in the stylus theme, and voila! Your theme is done :D

this is a very short look on stylus and web theming, you can find longer guides on this if you want a better explanation and look at stylus

That is pretty much all you need to know to get started on your web themeing journey :D

update: i have made an nord theme for youtube, you can check it out here