Two more “solutions” …
1. You could somehow put the image for displaying the category as background-image css property in includes/CatGridView.php, but I’m not a programmer and so I don’t know how to do this. In this way, the round corners would work in webkit-based browsers (i.e. chrome) and you could define custom borders as usual.
2. You could replace the border css properties with “box-shadow”. This way you can’t have custom borders (like double border), but it works. First, you have to remove all border properties, then replace following lines under light theme.
/* Light Theme */
div.cgview.light ul li { box-shadow: 0 0 0 3px #fff;}
div.cgview.light ul li:hover {box-shadow: 0 0 0 3px #300; }
div.cgview.light ul li div.cgback{background-color:#fff; }
div.cgview.light ul li div.cgtitle p a{color:#000;}