Hi,
The background color there is likely set by your browser. It seems that either the popup system which should be used when you click on the image has been deactivated, or there is some javascript error on the page leading to the popup system not working.
So, first, check that the "Image popup mode" setting in the HikaShop configuration is set to "shadowbox (external)". If it's set to "none", then the behavior will be what to describe.
If it's set to "shadowbox (embedded)", then it means your template needs to include the shadowbox library, and if that's not the case, you'll get the behavior you describe.
If it doesn't come from that setting, then it's likely a javascript issue. We would need the URL of a product page with the issue in order to check on the situation.