It's in the form.css template. If you can provide more details or a screenshot of what it is you are attempting, that may help.
I have a sidebar poll with WF and the Core Style from PixelExit. On Desktop the buttons looks like .. On mobile it looks like .. So i'm searching for the responsive part of these buttons. It appears also with the Login button, but NOT with the register button.
You may want to contact the add-on and/or style author to address that. I don't use either of those so can't help.
It's styling from iOS, we pushed out a fix in the latest xenbase 1.4.2 which should of addressed it. Let me know via the core style thread or pixelexit if you need further help.
