Sorry but what do you mean when you say CSS selector? And yes, you can add that to your theme CSS, either to the theme style.css file directly or maybe there’s a place on the theme you can place custom CSS.
You can just add the color property to the CSS above. So it should look something like this.
Yes that is possible. If you go to the Advanced Tab on your module settings, there is a place for a Class. You can place a class name, e.g., red-bg, then add that as a class selector on the above CSS. So it will look something like this.