Starting November 27th, 2018, there will be some small changes to the appearance of the Google sign-in page. These follow changes made earlier this year, which updated the sign-in page to match the Material Design principles used in other Google products.
Specifically, you might notice outlines around some entry fields, and changes to the spacing and styling of other text on both the web and mobile screens. The changes will start to take effect on 27th November 2018 - and may take up to two weeks to reach all users.
Please be aware of this design change. Google do this every now & again - nothing to worry about - it's not hackers!