The standard button-styles for links in the Responsive theme:
The stock colours are: blue, red, orange, yellow, olive, green, purple, pink, brick, gold, silver, gray, and black.
You use them like this (do this in the Text tab of the editor, make sure you’re using agle brackets “<” and “>”, not “>” and “<”. Note also that the button styling doesn’t show up in the edit screen, only when browsing the site):
<a class=”red button” href=”http://example.com”>Nice Button</a>