Built-in themes

[junegunn]

fzf currently ships with two base color themes, --color=dark and --color=light, partly based on my seoul256 and seoul256-light.

While the colors of fzf are fully configurable, it's tedious to customize each one of them.

https://github.com/junegunn/fzf/wiki/Color-schemes

Should we provide more built-in themes, similar to bat?