Empty button Error in wave accessibility checker

We need to pass the accessibility test with 0 errors/ 0 contrast errors in https://wave.webaim.org/ for a joomla site with joomdonation eshop module. We can deal with all other errors but one. It is shown in category and product pages and refers to some empty buttons used by colorbox module, which is used by eshop to show product pictures. The error can be inspected for example if you take this link which refers to a product page https://www.i-kosmima.gr/en/kosmimata-3/ginekia-kosmimata/wedding-ring-s-two-tone-3mm-antigrafi and put it in https://wave.webaim.org/ ( better inspect it in chrome browser and put styles> off to be able to click on the errors and see the code) . Colorbox is a javascript file and we need someone to modify it in order to change the empty buttons html tags. Freelancer to be hired need to have great javascript knowledge , html, php and have experience with accessibility site audits. We will provide a test site with admin pass to work on it and we want to have any code changes made to correct these errors , so we can make them in the real online site. THERE IS A DEADLINE UNTIL Friday JULY 28th...this is really urgent. So please don' t apply if not sure you can deal with this and have experience on these matters.