Modifying the .php or .css files

In my shopping cart module on the left-hand side I want to separate the Product Name from its Variant by displaying them on different lines.
For example in the screen shot below:

The Product is called 'Satin - standard Canon 190gsm'.
The Variant selected was 'Crop Image to fit paper size (recommended)'.

By default HikaShop strings these two together in one long line, whereas I want them on separate lines (perhaps by inserting a <br> into the .html.

According to FireBug I need to find a class called 'hikashop_product_variant_subname', so I expected to find .hikashop_product_variant_subname in a .css file (to format the text) or .html file (to add the <br>) somewhere as a start.

However I cannot find these files, so am I going about it the wrong way?
Many thanks.

Many thanks Nicolas,

I think I am starting to get the hang of how to modify my pages.

Regards.