Improve custom theme support.
With each new improvement to the custom theme the preview panel becomes more and more useless due to fact it only reflects the input colors. It's impossible to preview stuff like alerts and (in some MRs) inputboxes and faint text because those use computed colors (like midtones, desaturated versions, transparent) which are only known at the "apply stage", not "setup/preview stage". The proposal is to store colors in rgb(a) format instead of hex, and to calculate all the needed colors at early stage, so that user can preview all the stuff he needs right away.
Also colorpicker doesn't seem to support alpha channel so gotta make additional control for that.