Add inverted colors mode for light backgrounds

[extrawurst]

Add inverted colors mode for light backgrounds

[MCord]

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

[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)

[extrawurst]

see #65