Very basic CSS theme support

[tintededges]

My primary gripe with Nuclear is it's visuals, it just doesn't look pleasant to me for some reason, so I thought wouldn't it be great if it was possible to theme Nuclear with a custom stylesheet?

Another browser-based desktop music app, YouTube Music Desktop, enables themes by simply having an option for the user to add a CSS stylesheet that will be loaded on each page. Of course it's difficult for most users to make any meaningful change with this, but the idea is that people who know enough about CSS make and share full stylesheets like themes.

There are similar closed issues, #1277 and #1283, but they seem to be requesting a more complete theming feature unless I'm mistaken. I don't know much about Semantic UI specifically so I don't know if just having an option to add an extra stylesheet is similarly not possible. I assume it should be as simple as adding an extra html element into the header somehow, and if Semantic UI makes it hard to modify the header like this its also possible to add an extra header element inside the body and put it there.

[nukeop]

When choosing a UI framework I chose one with very poor theming support, that basically precludes any possibility of adding styles unless you do a lot of manual work. So while css could probably be loaded very easily, it would be hard to make a stylesheet without a lot of tedious work.