Remove hardcoded blue background color

[jantari]

Is your feature request related to a problem? Please describe.
It appears gitui uses a hardcoded blue color in some of its UI elements, such as the bottom shortcut row, the currently selected file on the top left and the diff header with the line information on the right. This blue color does not fit into most themes and may cause visibility and contrast problems with some.

Describe the solution you'd like
Use the terminals DarkBlue color palette instead of hardcoding colors.

Describe alternatives you've considered
None

Additional context
Screenshot of gitui with GruvBox Dark theme:

[extrawurst]

Hi @jantari thanks for your interest in gitui. You can customise all colors following this guide: https://github.com/extrawurst/gitui/blob/master/THEMES.md

let me know if there is anything you cannot customise to match your taste.

[jantari]

It is nice that the theme can be customized to take advantage of the 24bit True color terminals we have these days, but I think the default should be to use the terminals palette.

[extrawurst]

this is released in 0.8 now: https://github.com/extrawurst/gitui/releases/tag/v0.8.0