Hi wholthof,
Thanks for trying out Styles! It looks like Styles isn’t active on your site right now, so it’s difficult to guess at what the issue might be for IE7/8.
I did go and test some settings in the Styles demo for TwentyTwelve and things appear okay there for IE8, so my first guess is that the issue may have to do either with the specific settings you chose in Styles, or more likely, some of the CSS in your TwentyTwelve child theme.
As a first step in identifying the issue, can you try switching back to the default TwentyTwelve theme, then see if Styles still causes issues for you in IE7/8? If that resolves the issue, then there’s something causing a conflict in your child theme’s CSS. The next step would be to copy out half of your custom CSS at a time, save, and check if the error still exists. Do that until you get down to the line causing the issue.
If that step shows that there’s nothing wrong with your child theme, can you provide some specifics on what changes you set in Styles that caused the issue to happen? If you try those same changes in the TwentyTwelve demo, do you still get the same problem? Can you provide a screenshot of what you’re seeing?
Thanks for reporting your issue! By working through some of these items, we should be able to find you and others a quick solution to this problem. ??