I had a look at your site on Mobile, and the buttons seem to look good:
http://i.wpne.ws/TOTW
Have you managed to solve the issue by yourself?
If you still experience issues, you'll want to look at your theme stylesheet; it includes CSS media queries to define custom CSS rules for each browser size. You can search for CSS rules affecting the search button in mobile browsers inside each CSS media query, and apply the same rules to the subscribe button as well.
Let me know if it helps.
