On Windows it's Ctrl-PrintScrn, on Mac it's Command-Shift-3.

Question: how exactly are you "pasting in your button"? If you're doing it in a "Content" block, the block may be adding extra code around your button which could mess up the layout. Try pasting it into an "HTML" block instead. If that still doesn't work, it's probably some weird style in your CSS, which is not so easy to figure out as it could be any of a number of things. If you have firefox, try installing the "Firebug" plugin, which lets you click on things in web pages and shows you the stylesheet rules for them (which may help you figure out what's going on).