I came across this one and it seemed good enough to share, I hope Pivotal Veracity adds Gmail Themes to their rendering testing offering soon:

"The introduction of Gmail Themes-which allows users to change default background, text and link colors in their inbox-has made it even more difficult to code HTML emails so that they render consistently in Gmail. The problem arises when subscribers use certain Themes and marketers don't define the color of every background, text and link. Under certain circumstances you can end up with light text or links on a light background, or dark text and links on a dark background, making the email difficult or impossible to read.

For instance, here's a Nov. 21 JC Whitney email viewed in Gmail with the "Classic" theme (top URL) and the same email viewed with the "Contrast Black" theme (bottom URL):



 In this case, the preheader text and product names disappear (because it's black text on a black background) and the vertical navigation menu becomes practically unreadable (because it's yellow links on a light blue background). So what was a well-designed email with a good balance of images and HTML text is now partially illegible.

Based on the findings of the 2008 Retail Email Rendering Benchmark Study, my estimation is that approximately 40% of all retailers have the potential to be significantly affected by this rendering problem.

The solution is to define all your background, text and link colors."