I’m afraid adding transparency to the colors is only possibly using rgba: http://www.w3schools.com/cssref/tryit.asp?filename=trycss_color_rgba
I can’t really incorporate that into the Colors add-on without people losing their colors.
Your best bet for finding selectors is using your browser’s inspect element tool. In Chrome, right click and select “Inspect”.
In Firefox, you can use the Firebug extension.
Using Simple CSS is safe – your CSS will never be deleted when you update the plugin 🙂