Paint background color

**Is your feature request related to a problem? Please describe.**
When using terminal sharing applications like tmate or upterm, the people connected might have different terminal background colors. Thus, it will be unusably ugly for some of the users. This is because lazygit's background is "transparent", i.e. it uses the terminal's background color instead of the theme's.

**Describe the solution you'd like**
Please paint the background color instead of leaving it up to the terminal. Or maybe at least add an option/CLI flat.

**Describe alternatives you've considered**
Alternatively, a theme that works with both light and dark background (is this even possible?) might do it as well.