Here is documentation on starting your own theme: https://docs.nodebb.org/development/themes/
If you want to make small style changes, it's best to do it in the custom CSS portion of the ACP, instead of editing the theme files directly.
i want ta add a new topic button in navbar. which html file should I edit? I couldn't find.
@umuur which theme are you using?
@pichalite i'm using default one.
@umuur navbar menu is in this file
@pichalite thank you