I found that adding an InfoPath form web part to a page broke a lot of the custom branding we have added to our site. Inspecting the html, I noticed that a second reference to core.css had been added at the very end of the element, and after references to our custom CSS files.
In my case, the copy of the core.css that I wanted to keep actually included a querystring (core.css?somerandomlookinglettersandnumbers) so I simply disabled the offending core.css stylesheet as follows:
I created a new page layout for InfoPath forms and added this script at the very end of the tag (i.e. just before the closing tag for this section)