Can I integrate my existing form with a theme?

I have an existing form. I was wondering if I can choose a theme and integrate the two? 

You can definitely apply a theme to your existing form, please follow this guide: https://www.jotform.com/help/184-How-to-Apply-a-Theme-to-Your-Form 

I forgot to mention that if you want to apply one of the themes from the "Theme gallery", you will need to first switch your form layout to "All questions in one page":

oh super helpful!!! that last item is what made it work!! Thanks again!
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Welcome! :-)