Thanks. I've got it now.
For others reading this thread, that wasn't it. I did some further investigation on the .thumbnail class and the rule that was creating the effect is in /media/com_hikashop/css/frontend_default.css:
.hikashop_container .thumbnail:hover {
box-shadow:0px 1px 4px #ccc;
transition: all 0.2s ease-in-out 0s;
}
(I needed to activate :hover on the element in the new version of Firefox's inspector to make the rule show)
And I have now overridden it in template.css to achieve what I wanted.
Thanks.