Thanks for your inquiry.The s2Member Pro Login Widget comes with basic structural CSS/styles, but with plenty of room for style inheritance from your WordPress theme. Things like colors and font styles are always inherited from your WordPress theme, so any tweaks in this regard should be inside the style.css file for your theme Or, from your theme options panel, if your them provides you with the ability to enter custom CSS/styles.
If you have trouble with CSS, please DO post a link and perhaps someone in the community can assist you with CSS. Also see:
http://www.w3schools.com/css/ Firebug is also a popular tool among site owners. It can make tweaking things like this easier. See:
http://getfirebug.com/