Hey Mosi,
first of all, thank you for the purchase!
There is no option to change the overlay background color so far, but you can easily do it by adding a line of custom code: please go to the “Style: Add Custom CSS” tab of the Theme Options panel and insert this code:
.iso-masonry .image-post-overlay { background-color: #000000 !important; }
changing the “#000000” to the hex code of the color you need.
Regarding your question about the overlay size, could you please clarify what you meant by the “size” of the overlay? If you’re talking about the height, then it is automatic and depends on the amount of text inside the caption; you can also set it to a fixed height using some custom CSS, just let me know if you need that or something else.
Sincerely,
Andrew / Satori Studio