Let's face it, some people are just a tad too attracted by the shiny bar at the top of the post screen. The results can be quite ghastly, frankly, and when different themes come along all hell will break lose. So, here's my suggestion:

Add an option to ignore post formatting.

This would have the forums ignore colors, styles, fonts and sizes when displaying posts, creating a uniform look. Note that this would be an -option-, so people that like the colors can retain them.