Add setting for theme selection.

[Degit22]

Add setting for theme selection. Make 2-3 basic themes (light, gray, dark). By choosing a connected CSS. I understand that you can connect your own css, but for many it is difficult. And redoing the whole theme is harder than fixing a little. Ready to add additional themes if a setting is added to select which css is connected. And separated the current theme.

[tanriol]

I'd be okay with that, but I'm not that good at visual design to quickly create a few more full themes. The only part of the current Brief UI I've implemented myself was the "organize mode", and I think you'd agree it's borderline ugly. I'd be perfectly okay with integrating themes if someone implemented them (and there may even be a dark theme somewhere in the issues that I've forgotten about).

[Eragonfr]

You can probably take one of the theme suggested at the end of this issue #326 If you don't do a redesign it's ok, we can work with the current layout of brief ui. And I think that most of us just want a dark theme without any movement in the layout.