User281315223 posted
Without knowing more about your exact issue, such as how you are referencing your CSS files (to style the page, etc.) it's going to be very tough to troubleshoot this. I'd recommend opening up the Developer Tools (F12) within your browser and ensure those
files are being loaded as expected.
2017 should have no problems building / running an older 2015 Web Forms application, so I would suspect that it's an issue with some of the underlying files.