Hi,
First of all it's better to apply the css property on the following class "hikashop_product_description_main".
Are you sure that you are using the default frontend css file from HikaShop ? In Configuration > Display > CSS.
If it is selected, so you can apply properties in that file, to force the use of your properties, you can use the "!important" word after each css properties, like:
.hikashop_product_description_main{
width: 220px !important;
padding: 10px !important;
border: 5px solid gray !important;
margin: 0px !important;
}