Configurable commit tree highlight color

[anikiforov]

Is your feature request related to a problem? Please describe.
When using a light terminal background, the commit tree highlight in the commit view is hardly visible (and might not be visible on certain backgrounds). E.g.:

Describe the solution you'd like
The straightforward solution would be to make the highlight color configurable instead of hardcoding it here in the graph.go

Describe alternatives you've considered
Perhaps there is a way to programmatically determine whether the light or dark background / theme is used, and auto-select the highlight color accordingly.

Alternatively, it may be possible to use one of the theme colors defined by a user in their terminal settings as the highlight color.

Additional context
I am open to implementing this. Any suggestions and comments regarding the best approach for implementation are welcome.