Hi,

Try to add the following code snippet to the Custom CSS area in Theme Settings to fix this issue.

body .pswp__bg,
body .pswp__img--placeholder--blank {
	background-color: #FFF;
}

body .pswp__button:focus, 
body .pswp__button:hover {
	background-color: #F3F3F3;
	border-color: #F3F3F3;
}

body .pswp__top-bar .pswp__button:focus, 
body .pswp__top-bar .pswp__button:hover {
	background-color: #DBDBDB;
	border-color: #DBDBDB;
}

Regards