Your post is a bit old by now, but since I experienced apparently the same issue the other day, I thought I could post my findings to you.
I have a database where all my Tab Panels are alike. Because of that, I have so far only modified the default style. Then I decided to make another tab-panel with a different background colour, so I saved the default Tab Panel style as New Style, changed Apperance settings, Saved Changes to Current Style and lastly Saved Changes to Theme.
Back in the web browser, all my panels apparently lost their background colour. I say apparently, because what actually happened, was the Default style getting the same background colour as my new style for the Tab Panel, which insidently was the same as the colour of my background.
To cut a long story short, it all is (obviously) a bug in Web Direct, where settings in the New Style are applied to elements with the Default style in the web browser. I have tested this theory several times with consistent result. And I also experienced that if I added even more styles and modified them back and forth, things got more and more messed up, with no telling which style actually was applied in the web browser.
However, if I deleted all new styles and saved changes to style and theme, all got back to the Default Style settings.
My workaround is to set Apperance for the (non Default Style) Tab Panel, and NOT save changes to the style. As long as there are only one Tab Panel style, apperance in the browser is fine for those Tab Panels with the Default Style applied.
This error applies not only to the background colour, but all other Apperance settings too.
I have posted an Issue report on FileMaker Forum. http://forums.filemaker.com/posts/cdc9ac4cf3
Regards, Jon Riber Sparre