For the title bars and borders for really hdpi screens, look for Default-xhdpi and the various Arc-xhdpi themes in the "Window Manager" settings. I don't know of a way to automate switching between those, but fehlix probably has a desktop file that could do that.
By the way, "neofetch" in the terminal shows your current resolution and WM theme if you want to provide that.
Switch between different display settings
Re: Switch between different display settings
Thanks, but as I said, there is no need anymore. The normal settings now look good (enough) on the hdpi display. I don't really understand why, but somehow MX Linux has gotten better in automatically adjusting the display settings to the hardware.