The close, minimize, and maximize button for Mailspring doesn’t match my selected GTK theme on Linux. How do I fix this?
The buttons will only match your system theme if you choose to use the system window frame from Preferences > Appearance. By default the buttons (and the whole title bar, really) are “themed” by the Mailspring theme (which allows them to be dark in dark mode, etc.)
I think that part of the problem has to do with what’s reported here: Match Window Controls side to OS settings when using Custom Window Frame on Linux