It's an old post, but here goes if anyone else also struggled with this.
I added [hikashop_category_description_content] to the Custom.css (gets created once you fiddle with CSS in HikaShop) and configured the "<p>" style's format, as a nested element in the "<div>", as follows:
div.hikashop_category_description_content, .hikashop_category_description_content p {
font-family:Verdana,Arial,Helvetica,sans-serif;
font-size:30px;
padding-top : 20px;
padding-left : 5px;
clear:both;
padding-bottom : 20px;
text-align:left;
float:left;
background-color:#FFFFFF;
display:block;
width:100%;
}
If you inspect your CSS, the formatting is directed via the DIV to the "<p>" containing your category description, associated with the class[hikashop_category_description_content].