Your wish is granted! Someone added this a while ago. Sigs are now wrapped in two div tags, one with class pmsig, and the other with class pmsig-ID where ID is the user id of the poster. You can add on-site CSS in your user settings to change how they are displayed.

For example, I have sigs set to a font-size of 2px. "What's the point if you make them so small?" you may well ask. Mostly it's so that if someone accidentally gets their post content wrapped in the sig tags, I can zoom in my browser text and still read it easily, yet keep most sigs small enough that they are easy to ignore.