Although it is perfectly okay to post podfic without a workskin, many podficcers who post on AO3 choose to use workskins to make there podfics look better.
Workskins are compose of two parts: the CSS style sheet and the corresponding HTML.
AO3's FAQ on how to create your own workskins
Azdaema's guide to creating your own workskin for podfic
EosRose's guide to adding your own workskin for podfic as well as her own workskins
Reference guides and tutorials for basically all things HTML, CSS and more
An online space to write and test code
Tutorials and examples of more workskins to achieve different effects on AO3 beyond podfic