Hey @vuralparslan! It looks like this is actually a specific setting in your Site Styles, rather than something that needs to be adjusted with custom code. See screenshot:
The "Gallery Overlay Color" is the dark background that appears when you add a title and/or description to your gallery image. For more on this, you can see this in our guide here: https://support.squarespace.com/hc/en-us/articles/216034788
Hope this helps!