Please complete the following steps:
1) Go to the plugin settings page and open "Custom Code" tab;
2) Mark "Activate custom CSS code" checkbox in "CSS" section and insert the following code:
#fancybox-content, #fancybox-title, #fancybox-outer {
background-color: #000000;
border-color: #000000;
}
3) Save changes.
This article related to Galley by BestWebSoft plugin
Additional resources: Gallery - User Guide
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