The DynamicCSS plugin is used to place a drop-down in the toolbar to allow applying CSS classes to the selection. The drop-down gives access to CSS styles that Xinha has access to. So, for example, if you use the code below,
config.pageStyle = "@import url(style.css)"
then whatever styles set in "style.css" will be available in the drop-down (hence the "dynamic" part). Note that a full URL is required for import url.
It was originally developed by Holger Hees.
In Xinha you may find the Stylist Plugin offers better functionality, at the price of a little more screen realestate.