Relative font sizes in the skins (ie. <font><size>+1</size></font>) would be cool. TOOLS->PREFERENCES->APPEARANCES would seem a logical place to set a default font size for skins.
As far as that goes, having (say) 5 font settings (1-5, with 3 being default), a skin coder could do something like:
<font id=1/> and user settings (color / boldness / italics / underline / typeface) could be taken from the the appearance dialog. Naming the numeric sequence (small, link, link_visited, normal, subtitle, title) instead of numbering would clarify for the user, what exactly they were changing.
This logic could be extended to globally configure <section> colors, speed buttons, rating images and other such things (skin background image, PVD info pane background image), separator images, which columns to display in filmographies, selection of an external viewer for images...it goes on and on...an option to display an image with a link...set sizes for screenshots and posters.
The only drawback I can think of is when a user likes the whole appearance of a skin, but doesn't have the APPEARANCE settings to accommodate this. This would require the setting of defaults in the skin that could be overridden by the appearance settings (ie. "use skin defaults" | "user skin settings"), or an external file that is included with a skin and loaded automatically by PVD. Now that PVD scripting supports files, it may be possible to write a script that does inline conversions and rewrites the skin....hmmm...might have to try that...