The standard class name in DotNetNuke for Link Buttons is CommandButton. If you change this however, all Link Buttons on the entire site will change.
You can replace the Form Master default submit button by adding your own which can be a Link Button, a Standard Button, or an Image Button.
When adding your own, you can use your own class name, and put the CSS Rule in a .css file. I generally like to use portal.css for this type of use.