Thumbnail Styling different than other images

Hello guys,

https://www.dailybuzzer.com/britney-spears-in-pussycat-dolls-video

On this post you can see that the first picture added has black borders, but the other thumbnails, linking to the full images – have grey borders, where do I change these styling features for linked thumbnails?

they turn grey whenever I make them linked.

Thanks,
Heat