Selected tab is hardcoded to white and cannot be changed via theme

[star-szr]

Describe the bug
The selected tab is not overrideable via the theme, and in addition it is hardcoded to white since around v0.11.0-rc1.

I recently have been trying a light theme, that's how I noticed this.

I will be submitting a PR to at least remove the hardcoding to white, and if you'd like I can also restore the previous coloring of selected_tab but probably needs discussion.

To Reproduce
Steps to reproduce the behavior:

1. Change your terminal emulator to a light theme
2. Launch gitui
3. Notice the white-on-white

Expected behavior
I would expect the selected tab to at least be neutral colored, or to respect the theme.ron settings.

When editing theme.ron I thought this was controlled by selected_tab, but I tested v0.10.1 and it seems like it's been neutral colored for quite some time (since v0.3.0 when theme overrides were added).

Screenshots

v0.15.0, light and dark with no theme overrides:

Context (please complete the following information):

• OS/Distro + Version: Pop!_OS 20.10
• GitUI Version: v0.15.0
• Rust version: Not sure, but probably not relevant in this case. I installed gitui via nixpkgs but I can reproduce the behaviour by building gitui from source.

Additional context

Screenshots from v0.2.6 (before initial color theme support was added)

Screenshots from v0.10.1 (before the selected tab was hardcoded to white)

[extrawurst]

closed via #722