Configurable commit tree highlight color

**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 ](https://github.com/jesseduffield/lazygit/blob/f085d10c465c3a80be492390292b787f037c6f91/pkg/gui/presentation/graph/graph.go#L33) 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.