Here's a quick tip for adding a background color to the .tabularBoxTitle > header element in WSF 5.0 which should save you a lot of trouble (which I've been going through today because of this).
If you just want to change the background color of the tabularBoxTitle header, add the new font color through your style's CSS Advanced Settings and do not change the font colors in the color palette. If you change the font colors in the color palette, you might end up with a light color font on a light color background where you do not expect it.
Your added CSS should look something like this:
Again, leave the settings in the color palette for wcfTabularBox alone.