Add setting for color scheme / theme

[dmlls]

First off, thanks for the great work on the app!

This feature request is related to #364. Since currently it's extremely hard to read any updated values when reviewing a changeset in dark mode (see picture below), it would be great if at least it was possible to manually set the color theme (auto / light / dark), since in the light theme the values are readable. Currently, the app sets the color scheme automatically based on the browser preferences.

This setting would go into User preferences -> General.

[cdauth]

Good catch! For now I have fixed those colours so that they are readable again. I will implement such a setting soon.

[dmlls]

That's already great, thanks a lot for the quick fix!