Allow custom color hex codes to be chosen for themes

[Geroyuni]

One of the things I appreciated about Zen most was that I didn't have to trial and error my way into getting the exact colors I wanted for my theme. The newest version undoes this customizability option.

[Travisyard]

Additionally, the custom color/gradient picker simply cannot select all colors. It seems to have a subset of muted/pastel colors that it allows you to choose from. The closest to a bold red I can get is #f36666 when I really want something more like #e00b05.