The greatest concern of any good webmaster, while changing the themse/css of the site, is if the new UI will work properly on different browsers and on different screen resolutions. Browsershots proves really helpful in such a situation. Choose the different browsers and screen resolutions for which you want to test your site and submit the page url. You will get the screen shot after 3-4 minutes. You can opt for a paid mode to get faster service.
You can see the output for one of the pages of binaryday here
Good TryA tool to test your website/blog on different browsers and screen resolution250|just some value|9