The "design" is controlled by styles in a CSS.  Most skins have a selector in the skin's main stylesheet (i.e. skin.css)such as .logintxt and various link forms of that selector.  You should be able to find something like this:

You can edit to your heart's content.  The two main paths are to edit this selector or create a custom one and edit your skin file to refer to the new class.

BTW, if the skin.css file doesn't work, check the default.css.  This may be the final stylesheet applied and may overwrite portions of skin.css.