For all the effort you put into building your site, the most important thing to your users is how your site looks. Managing the inconsistencies in your theme for each browser, for each module, and for user selectable themes can be very challenging.
First you should familiarize yourself with basic Cascading Style Sheets (CSS) concepts. Review CSS Discuss or HTML dog for CSS resources. A good overview of the power of CSS can be seen at the CSS Zen Garden.