Okay, I misunderstood where you want to do this. To add a border around category thumbnails, Add this line of code to your main.css
Code:
/*
Adds border around category images
*/
div.subcategories img {border: thin solid #000000;}
and to add a border around featured products, and sub-category thumbnails add this code to main.css
Code:
.products-list img{border: thin solid #000000;}
add a different stlye by putting your css in the curly brackets.