Hi @Yan_Rudenko, the best docs to start with are probably this tutorial:
As you mentioned, Plugin Outlets are the best way to customize the UI. But these are not available everywhere. Only certain parts of the UI are designed to be customized.
Manipulating the DOM using regular JS will cause errors, because our rendering framework will lose control of the elements it rendered to the screen. It’s best to stick to the supported plugin outlets and other Discourse JS APIs (also covered in the tutorial).