I've noticed that with Mint-Y-legacy, the window controls are consistent across Metacity/GTK, regardless of icon theme. This appears to be caused by a reference in gtk.css to an "assets" folder stored alongside gtk.css, which stores the window control icons. This reference drives the consistency.
Presumably, it would be possible in current Mint-Y themes to remove the gtk.css reference and dependency on Mint-Y icons, and replace that with an "assets" folder ensuring consistent window controls.
I've noticed that with Mint-Y-legacy, the window controls are consistent across Metacity/GTK, regardless of icon theme. This appears to be caused by a reference in gtk.css to an "assets" folder stored alongside gtk.css, which stores the window control icons. This reference drives the consistency.
Presumably, it would be possible in current Mint-Y themes to remove the gtk.css reference and dependency on Mint-Y icons, and replace that with an "assets" folder ensuring consistent window controls.