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CSS for customizations can be added to your Wiki's MediaWiki:Common.css page.
I'm a nub when it comes to coding though and have no idea what to put down.
You can add something like the following in your MediaWiki:Common.css page.
li.gallerybox div.thumb { background-color: #53708e; }
The # value provides the colour value. You can check those values and their corresponding colours from HTML Color Picker.
A quick method to make CSS changes on a website is to use your browser's developer tools.
To do this, while on your Wiki page, press Ctrl + Shift + C, which will open the element select tool. Click on the webpage element you want to make changes to. This will in-turn open the element IDs and CSS style definitions (like font, colour etc.) for that particular element. Use these definitions in your Common.css page and customize them as required.