sebastix on Nostr: Theming can be quite complex, so it takes some effort to get started. ...

Theming can be quite complex, so it takes some effort to get started.

https://www.drupal.org/docs/develop/theming-drupal

But you can alter / hook into your current theme by making a subtheme.
https://www.drupal.org/docs/develop/theming-drupal/creating-sub-themes#s-generating-a-new-sub-theme-using-drush