@blablah said in Is there a way to use custom chromium themes in this browser???:
It does here: https://help.opera.com/en/opera36/personalize-the-browser/
Yes. That's an old help page for the very old Opera 36 where the ability to change the background of the start page was called "Themes". But, it didn't do more than that. In later versions and now, that same feature is just called "Wallpapers". As in, you can change the wallpaper for the start page. Opera can't do anything more than that besides switching between dark and light mode, which is under a Theme category. But, there are no other color modes and no support for Chrome's themes.
Now, Opera GX on the other hand does have some theme support. Just click the Easy-setup button at the right of the address bar and you'll see the different colored themes you can select. And, you can customize the colors. And, you can still set a start page background like regular Opera. Chrome themes are not supported though.
