Change your Custom CSS so the border is #999 instead of white so that you have a darker border on all the other pages. Then repeat the code, with the border set to white, prepending the selectors with '.homepage'. This will ensure the homepage is white instead.
Let me know if you need more specific advice.
Your site using using a Brine-family template btw, which includes what Squarespace refer to as the 'Custom Template' (sites made by answering questions instead of picking a design).