If you are using the style sheet templates I don't think there is an easy way of changing all of your galleries from using one template to using the next. However, what you can do is create your own style sheet then copy and paste the code from the template style sheet into your own. Then you link all of your galleries to your own style sheet and whenever you want to change the style of all your pages you can just edit your style sheet or copy and paste code from one of the templates into your style sheet.
In order to actually get the style sheet code from the templates, first go to your edit gallery page and click on the sample stylesheets link. Click the link for the template you like and view source on the page. At the top there should be 2 <link rel="stylesheet" ...> tags. Copy the url from the href attribute that starts with
http://css.pbase.com/styles and view that url in your browser. The page that opens will be the code you want to copy and paste into your style sheet. Hope this helps.