Since your whole gallery from what I can tell uses the same style sheet you just have to change that style sheet by editing it. You do not need to make a new one then change them all to that.
Just go to a gallery edit screen, where you pick your style sheet you will see a edit style sheet button. Go there, then on the next screen hit edit again on the style sheet you want. Make your changes or copy and paste the whole new style sheet over the old one. Update the CSS and you are done. All gallery's using that style sheet will be affected.
And once you get used to using something like webdev, making changes to your css and applying them to pbase becomes very easy and simple.
PS. Very nice gallery