Stylesheets are the best way to control the appearance of your site. They allow you to make changes to your sites appearance quickly and universally over your entire site. CSS helps keep your HTML cleaner by separating out the style from the markup, and there are a lot more things you can do with the appearance of your site in a stylesheet than you can with just HTML.

As you are editing your templates you will see class="something" in a number of the html tags. This class item goes to the style sheet and tells your browser how this html item is suppose to appear in your page. If you want to change the appearance this something, add
.something { font-size: 20px; font-weight: bold; border: 1px solid; }
to the bottom of your stylesheet (Admin -> Themes -> Manage Templates -> Stylesheet). In this example, it makes all items of class something appear in 20px font in bold with a 1 pixel solid border.

There are a number of great places on the internet to learn more about Cascading Style Sheets. This is one of the great references site you can find. WDG Cascading Style Sheets This may not be the best place to learn about creating Cascading Style Sheets, but as you are making up your style sheets it is a place to go to reference the possibilities. There is also software out there to help you with your style sheets. Find what works best for you.

One of the places you can start learning more about CSS, is just to load one of the style sheets into your online editor. Change some of the items and see what happens. Move to different pages and see how it has changed the appearance in all your pages.