I have styled the ‘Subscribe’ button but when viewing on a mobile (iPhone 6) the subscribe button has lost all of the custom CSS. Also, when I view on my Mac at the same resolution as the iPhone the button appears as it should.
Probably the button is not loosing the style but getting a specific style for low width devices. Those rules could be inside your theme css. Or your rules are targeted only to large screen and not applyes to the smaller ones.