Talking generally. You can build a system which accepts new source code and styles and applies them.

Think about modding games for example. They are created without access to source code. The developers provide the ability to add new functionality to the system without providing the source code.

Heres the source code for Chromium upon which Chrome is built

https://github.com/chromium/chromium

You can modify chrome, edge, firefox. Plenty of themes and extensions exist. Just because its not a simple css file doesn’t mean it can’t be changed.