I’m not exactly certain I follow. You can change the image opacity and change the panel and overlay color, see the doc page but the panel and overaly itself are not set to any opacity.
… or …
Do you mean how the active panel will be a lighter tint than the selected color? If that is what you mean, that is handled in the
gridder.js file on line 210. If you make a change keep in mind the
gridder.min.js is the file being served.
Does that helps!