If you want to experiment with the Gallery Manager theme, I would recommend that you set up a second Showkase site in a completely separate folder to your original Showkase site (so that they do not interfere with each other).
The Gallery Manager theme is designed to manage galleries only. It is not designed as a complete web site theme (like Boma, Kosel and Maribo). As such, the Gallery Manager theme supports only Gallery pages and Gallery Index pages. It does not support About, Basic or Contact pages so as soon as you change to the Gallery Manager Theme, all About, Basic and Contact pages in your site will be hidden. You can always change back to a regular theme and all About, Basic and Contact pages will instantly be reinstated without having to recreate them.
Gallery pages within the Gallery Manager theme display only the gallery itself. No header, footer or other page content is displayed.
As each page within Showkase is given its own folder in the Showkase root, you can embed your Showkase-created Juicebox galleries into any other pages on your domain using the baseUrl method of embedding (with the baseUrl pointing towards the Gallery page's folder within the Showkase site).
The Gallery Manager theme is similar to svManager (an older gallery manager which has now been discontinued) so anyone migrating from svManager might find the Gallery Manager theme familiar and useful.
More about Showkase in relation to svManager can be found in the Converting from svManager support section.