Discourse Code Modification for personal use

Is there any guide for making changes to discourse codebase for adding features or changing UI.

I don’t want to use Theme Component or Plugin API

It’s silly not to want to use theme components. They are always preferable unless a plugin is required. See Beginner’s Guide to Creating Discourse Plugins - Part 1


Editing the code directly via a fork isn’t really supported.

If you choose to go down that path we won’t be able to offer you any assistance.

The theme and plugin APIs are there to help you make changes whilst still being supportable. Maybe tell us more about what you’re trying to achieve, instead of how you want to achieve it.