Hello Hannah,
Happy to help!
When I publish your course, I see the same borders in the published output visible in the Slide Master view. Here's the published web version of the slide:
It might not be obvious in the screenshot, but the top border appears thick as well when viewed on my end.
And here's a link to the same slide, published to Review 360. The same amount of the blue background is also visible in the link.
Since the issue might be specific to your installation, try repairing Storyline 360 to see if this helps. If the issue persists, please open a case with our support team here so we can continue investigating the issue.
