Editing style of nextGen gallery

I’m able to activate and edit the default plugin style sheet for my next gen gallery, but I can’t seem to go beyond that. For example, I really wanted to edit the overlay color and opacity of the thickbox overlay. Another post on here mentioned editing the relevant section of the thickbox.css which is in the “WP Includes” file, and I did that, but it had no effect on the appearance of the thickbox overlay. Then I switched from thickbox to shutter. A different post on here told about going into the plugin editor and editing the shutter-reloaded.css but when I tried to do that, the shutter-reloaded.css says “inactive”. Also, when I try to change styles from default to “Black Minimalism” or whatever, I don’t see any change in how the gallery displays images. It’s like I’m stuck, and can’t make the changes that I keep on seeing recommended. My site is here.