I'd like to submit a feature suggestion: please consider adding support for custom CSS modding for the browser's user interface, similar to Firefox's userChrome.css system.

Currently, Opera's UI is strictly locked down. While the built-in themes and accent colors are nice, power users are unable to deeply customize elements like the address bar (URL bar), tab layouts, sidebars, or hide unwanted interface icons.

What I am proposing:

An option (even hidden behind opera://flags) that allows Opera to load a local custom.css file to override internal UI styling.

The ability for users to tweak UI padding, fonts, roundings, and hide specific buttons/elements on the top bar or sidebar.

Why this would be a great addition:

"Power User" Appeal: Many users sticking with Firefox only do so because of deep UI modding. Bringing this to Opera would make it the undisputed best browser for power users.

Flexibility: It lets users clean up clutter and adapt the UI perfectly to their desktop setup and personal workflow.

Thanks for considering this! I hope to see some level of UI customization in future builds.