Multiple galleries - can you edit the display page for each gallery individually?

In a group with multiple media galleries, I am using the body/right margin layout for the "home page"/files list for each gallery, and want to include a different generic content widget in the margin of each gallery's home page.

When I Manage Theme/Edit this Page to add a generic content widget on the page listing files in one gallery, that same widget shows up on the display pages for the other galleries. I can't seem to control the page layout for the files list in each gallery separately, even though they have different URLs. Is this by design? It's frustrating.