Extraneous horizontal scrollbars

I'm helping a friend put together a gallery view that opens the images with a Colorbox trigger. Horizontal scrollbars are appearing only on portrait-oriented images that are as small as 450 pixels wide [up to about 600 pixels wide, I think]. I thought this was an issue of the cboxLoadedContent div needing a min-width, but no matter what I tweak from the css side [#colorbox, #cboxContent, #cboxLoadedContent, etc], the scrollbar persists, even when the pop-up is wide enough. It looks like my css styles are being overridden by the module's own stylesheets, which of course I don't want to tamper with. Any insight would be greatly appreciated.