See this article for a full description.
To reproduce it, make a Theme and change the icon color to White, then make anything this an icon, and open it in IE7 or IE8. (Or IE9 with IE7 or IE8 mode on).
The description is best summarized here:
"When you as much as click into the DISC COLOR and DISC OPACITY settings, even without actually changing these settings, and then download the default theme, the background color in the following code in the .css file is changed from #666666 (the default setting) to #FFFFFF."
This has been posted here already, but it was declined as a jQuery Mobile bug, not a theme roller bug. I was asked to post it to jQuery Mobile's site, and I did, and they rejected it as a Themeroller bug, so I'm posting it here again.
Fixed icon issue #120 & #121
I'm so sorry for the misunderstanding. I've identified the issue. It should be fixed now.
No stress. It's the internet. Odds of us meeting again are like:
1 / (population of the human race * probability that a human is on the internet)