Update colors of light and dark scheme

This MR updates colors used in the light and dark scheme of the app.

Although I think I have a sense for aesthetics, I realize that I'm not a design professional. For the future we could get somebody involved who has a more professional view on this, but for now I think this could work.

Please review both schemes and tell me, if anything doesn't look right for you, I might have missed some bits and pieces.