White margins in the actual thumbnails. Need black to go with theme

-- HikaShop version -- : 3.4.0
-- Joomla version -- : 3.8.7
-- PHP version -- : 5.6.35
-- Browser(s) name and version -- : Firefox 58.0.2
-- Error-message(debug-mod must be tuned on) -- : My theme is black and while I have set the background color to Black, the images when converted to thumbnails from an upload image, has white margins (borders). Where to I edit the settings to make them black?

As you can see in the attachment. The extra margin space is white. And I need it black. Thank you