Problem solved. Using even another new browser as Firefox and it looked ok again. The thing is that all the browsers used the cached styles from the old ISPConfig. Clearing all history/cache/etc from the browsers and now they all look great.
Why the browsers didn't notice that the style file was new and instead loaded the cache is for someone else to figure out. But the same thing happened for Chrome and IE.
Maybe a hint as to when updating ISPConfig you might have to clear the cache is needed for future reference...