Just so you "themers" all know; the next version of Sigil has added the ability to define a custom user stylesheet that is applied to Preview and Book View (which cannot be styled via the .qss file). Those with the ability to compile Sigil themselves can check it out now. The rest will have to wait (hopefully no longer than the end of July). Just create a file named "custom_preview_style.css" in your Sigil Preferences directory and add your custom css to it. Sigil will load it whenever it launches.
Just remember that you're on your own should an epub's css decide to pick a fight with your custom user css.
