It could be in a number of places, listed in order of probability:
- the advanced stylesheet
- Custom CSS theme files / is the same theme selected
- (less likely) injected css into perspective styles
- (almost certainly not) any of the HTML text files located under
<install-directory>\data\modules\com.inductiveautomation.perspective\injectables - (hopefully not) injected CSS via JS injection or otherwise using the Markdown component
Look for css selectors for ia_popup or similar