Make CP color customizable

[acalvino4]

In general Craft CMS has good whitelabeling support, but I would be really nice if we could change the main brand color (the bright red one used on all the buttons). Even just specifying the color via css custom properties in the source would be great, as that would make it easy to override. Currently that color is defined in the css as

.btn.submit {
    background-color: #e12d39 !important;
}

and that !important is kinda inconvenient to try and override. This is just one example of the way colors are specified in the cp css; there probably are more. My proposal is simply to specify these colors (and I suppose the other cp colors as well) as css custom properties. The bonus level would be if there was a setting for brand color in the General Settings section of the CP.