Custom html form GUI

I have a GUI design for a calculator that I need to build in XHTML. I would like to use semantic markup here which (I assume) would use form elements to compose the structure of this calculator. After all there are buttons on it.

Is there a way to completely override the visual representation of form elements in browsers? If not then I’ll have to use some other tag selection which is not in the spirit of compliant code, but I have to make this thing look exactly like the design…

Thanks.