[BUG] Selected / DarkerSelected colors look bad on light mode terminals

[godlygeek]

Describe the bug
After configuring delta to use light mode, diffnav looks pretty good in a light mode terminal, except that the footer and the background for selected tree nodes are still very dark. There seems to be no way to configure these colors without recompiling diffnav. I'd be happy enough if they were just exposed as config options.

To Reproduce
Steps to reproduce the behavior:

1. In your .gitconfig, set:

[delta]
	light = true

(This shouldn't be necessary, but it works around #119)
2. Pipe a diff with multiple files in it to diffnav

Expected behavior
Colors to look consistently appropriate for light mode.

Screenshots

Notice the black footer, the black directory headings in the diff pane, and the too-dark background for the selected tree node.

Desktop (please complete the following information):

• OS: [e.g. Mac] Mac
• Version: what version are you using? 0.11.0
• Terminal Emulator: ghostty
• Using tmux: same with and without