I'm using android, sometimes in joiplay, sometimes in android ports.
I don't know about overruling the global setting, I just set the opacity to 45% and choose the vulcano theme, then click on save on the top right side and name the urm file, then I copy both the urm file that I named and the rpa file to the game location, and load the urm file after opening the game, then I can only find the textbox changes that I made, the other settings don't change, they become the default 10% opacity and the default theme is applied.