Find: /includes/templates/YOUR_TEMPLATE/css/stylesheet.css Start by opening the style sheet in your favorite text editor:
All of the pages are broken into smaller pieces called "classes" and each class has a style. The class styles are used to control the look of your fonts, colors, text size, borders, background images, etc. Change the colors by substituting standard HTML color numbers for the text and background colors. Change the text size by increasing or decreasing the size, change the typeface by using a different font name.
To remove a CSS element, such as a border, simply comment out the line you don't want to use with a "/*" (slash, asterisk) at the beginning of the line and "*/" (asterisk, slash) at the end of the line. After making your changes upload the stylesheet to the directory, refresh your browser and admire your handywork.
For more information on Cascading Style Sheets search using Google for links to tutorials and references.