After making changes to page designer, payment page keeps old design while the rest of the webforms display the new design.
This is caused by custom code within the Page designer. To resolve:
Go to Web > Page Designer
Click Next to get to Colors & Styles
Click Advanced colors and styles
Click Edit style sheet
You'll need to remove blocks of custom code and test clicking Submit on the Configure Content tab. If you get an error, the code causing the issue is still present. If it saves correctly, the portion of the code that caused the issue has been removed successfully.