Popbox CSS Styling Overridden

I’ve inherited a WordPress website that was developed by another company. Any further development involves picking through myriad stylesheets complete with thousands of lines of code trying to find offending commands. But this issue with some Popbox ‘more information’ popups has me stumped.
When I first load the page the Popbox code is exactly as I entered it. However when the Popbox link is active the code is modified. A <style=””> tag (element.style) appears that specifies the Popboxes size and positions it at an absolute position on the page. My stylesheets specify relative positioning and has rules for different screen sizes, but this command is being completely overridden.
I suspect that some Javascript or JQuery voodoo is at work here (I’m a designer, not a programmer so I could, of course, be completely wrong). Can anybody give me a steer as to where this tag might be generated from so I can get rid of it? (as you can imagine it’s playing hell with Responsive Design).
Any help greatly appreciated. Thanks.