Hi,
Sorry for the misunderstanding!
To do it, please go to Customize > Additional CSS > copy and paste the following code into the box > click Publish at the top.
.ct_gallery_list .col-md-4 .shade {background-color: rgba(20, 28, 197, 0.4)}
20, 28, 197 is your color code, and 0.4 is the transparent value.
Best regards,
Jean