fix dark mode header color; settings not topmost

[molsonkiko]

Previously header rows and the row header column did not have the same coloring as the rest of the data grid when applying dark mode or another theme. This has been fixed.

I also made the settings form not topmost, because I didn't like how it got in the way of pop-up messages (e.g., the message that asks if you want to scan the whole directory tree) and the navigation form. It will still display on top when focused, as normal.