Fix <select> dropdown background color

[erdnaxe]

When using a dark-mode Bootstrap variant with a Chromium-based browser, <select> elements contain white text on white background.

This is due to main.scss overriding the background super property, discarding Bootstrap background-color property.

Also applying .form-control on select elements is non-standard and was contributing to this issue (https://getbootstrap.com/docs/5.0/forms/select/#default).

Before/after this patch:

[ColdHeat]

I believe you need to rebuild the CSS files here. If you can do that I can merge pretty easily, if not I'm happy to build it for you as well.

[erdnaxe]

Thanks! I was unsure about whether I should rebuild the CSS here as it could cause merge conflicts.