The following documents outline advanced topics for customizing elements of checkout page design. Please read the following statement about support limitations and consequences before performing such customizations.
Limitations and Warnings
Use extreme caution when making changes to the values in the Checkout CSS file. Changes to CSS attributes can have a dramatic negative impact on the display of your checkout pages and should be attempted only by merchants with advanced knowledge of CSS. Yahoo! Customer Care cannot assist in debugging faulty CSS entries. Merchants encountering problems with their CSS files are encouraged to revert to the default styles, modify design styles through the Checkout & Registration Manager interface, or seek guidance from a third-party developer.