I would love to be able to skin/customize the color of the buttons and the other grey parts of the GUI like the Details tab, panel dividers, column names, etc.
I know that this can be changed using High Contrast themes in Windows but those themes change how everything else looks including webpages (which I don't like).

There's a reason "custom colors" only affects the parts inside Tixati within its control.
The development team thought that following the default OS colors whether inside Win or Lin is a good idea.
Settings -> User Interface -> Custom Colors

I too would like to see this option. Following the default OS colors is a good practice, but in sometimes I like for the application to have its own theme. Works great in Spotify and several of the Adobe tools, among others.