Add support of Custom themes

[t3dium]

An element style dark mode would be nice since I think the current dark modes colour scheme looks a bit outdated.

Can we have a theme with: text background #15191E, sidebar #23272C, and the people+home sidebar to be #2A282B

Also, an option to change font size would be nice, by default the font looks quite massive

[ghost]

It would be better to have the custom theme feature instead of asking for a theme to be added to Cinny.

[GTcreyon]

How exactly would this be implemented? My understanding of Tauri is limited, but I'm not aware of any way to import custom CSS from the filesystem without CORS issues.

[Kommynct]

You could cheat and use the element themes backend, and just assign the css selectors to the same names as element.

This would also make the themes interoperable, and bring all of the element themes to cinny.

[archeite]

How exactly would this be implemented? My understanding of Tauri is limited, but I'm not aware of any way to import custom CSS from the filesystem without CORS issues.

Maybe a text box to paste CSS into? Or maybe how Element (Labs) does it with theme URLs? (https://github.com/vector-im/element-web/blob/master/docs/theming.md)