You can use CSS with your help project. You do this by adding a style sheet to the project or by using the code editor. You use standard CSS in the window for CSS stylesheets.
One thing that will not work is opacity in the help viewer. The web browser control that comes with Visual Studio as of this writing does not do opacity correctly. Which is a real shame since opacity can be used to make some really awesome looking pages.
You can get around this by adding the opacity as a background image but that's good for applications. Not so much for help documentation.