Add inverted colors mode for light backgrounds

[piegamesde]

Is your feature request related to a problem? Please describe.

Currently the main text color is yellow. Which doesn't read well on a white terminal background.

Describe the solution you'd like

A CLI flag to switch to a dark color theme for better readability.

Describe alternatives you've considered

• I don't know if it's a good idea, but simply inverting all colors might be enough
• Full blown (custom) theme support
• An environment variable or a configuration file are viable alternatives to CLI flags

Additional context

[extrawurst]

Hm that is a more complicated issue. But it is definitely important to find a solution for that since everyone uses different colorsets - so it is essential that the solution scales

[extrawurst]

Similar but less challenging than #57:

1. refactor all Style objects that are generated on the fly all over the place to be statics first, because that allows us to figure out what are the common ones and derive the theme of gitui
2. introduce a new struct Theme that holds values for all these values
3. initialise this default Theme to look like it is right now
4. use crossterm feature serde + ron-crate allowing us to save/load it in a decent format (*.ron)
5. load the theme on startup if the file exists / dump the theme to file to make sure changes are reflected (additions)

[MCord]

I am going to try to refactor the code and let you know how it goes.

[extrawurst]

see #65