Dear Sitepoint

I'm thinking of buying me into a premium membership of a website that offers templates. Why? Because getting a new website perfectly operatable for aswell Chrome, old IE, new IE, Firefox AND most important different mobile devices is a bit too hard for me unfortunately..

Therefor my question: Are any of the websites that offer premium templates good enough for all that crossbrowser visibility AND user-friendly and good-looking on mobile devices? Are you against such sites and why?
If so, could you recommend me such a site?
Any other tips?

Thanks a lot in advance!
Kind regards
Maxx-iT