Below is how to fix the Category Boxes so they line up. You only need to add 1 line of code to the CSS for the height. It's better than white space being displayed due to different lengths in descriptions.

Find the below code in \includes\templates\responsive_sheffield_blue\css\stylesheet.php

/*bof category*/
#categoryDescription{margin:15px 0 30px 0;font-size:100%;}
.categoryListBoxContents img{margin-bottom:20px;}

Add the line with height:150px to your code. It is around line 387. You can adjust the 150 to your needs.

/*bof category*/
#categoryDescription{margin:15px 0 30px 0;font-size:100%;}
.categoryListBoxContents {height:150px;}
.categoryListBoxContents img{margin-bottom:20px;}