One place to start is in the documentation. If you go to:
SAS(R) 9.1 Output Delivery System User's Guide -->
HTML, Printer Family, and Markup Languages Style Elements and Their Inheritances
you will find the "official" documentation for inheritance. Generally speaking, however, there are a couple of quicker ways to track down the style elements that you might need to change. Once you know the element you need to change, then you can go to the doc to find its inheritance chain.
One method involves the use of creating an HTML file and then looking for the CLASS= attribute in the HTML source (that is your link to the "lowest level" Style element that controls the style). Or the other method is to use a diagnostic tagset, like style_popup to find the style elements.