Thumbnails to big in Issue postlist

I'm already stuck for hours on the following problem. The thumbnails of my post list are too big. I use the issue theme.

This css class is the problem:
div#page .upfront-output-region-container .upfront-output-module .posts-featured.uposts-object li .thumbnail > a, div#page .upfront-output-region-container .upfront-output-module .posts-featured.uposts-object li .thumbnail > img
This is the of the thumbnails, it has width 175%.

admin-ajax.php line 10330 (not sure if this changes for everyone)

How can I solve this? I only have it on the full desktop view, not on mobile.