This is desirable so that you can use a plain slideshow skin and allow the basic colour schemes and fonts to be defined in portal.css or the current skin css for easier per-portal style integration.
This fix applies to viewer.aspx and slideshow.aspx and can be recreated by copying code from the core dotnetnuke default.aspx.vb

1. In the .aspx itself, add
<asp:placeholder id="CSS" runat="server"></asp:placeholder>
(before the link instruction for the gallery css

2. In the .vb add imports to the header as:
Imports System.Web.UI.HtmlControls
Imports DotNetNuke.Common
Imports DotNetNuke.Common.Utilities

3. change the page inheritance from
        Inherits System.Web.UI.Page
to:
        Inherits DotNetNuke.Framework.PageBase

3. copy the ManageStyleSheets function from dnn default.aspx.vb into the .vb

4. in Page_Init, add:
            ' add CSS links
            ManageStyleSheets(False)
            ManageStyleSheets(True)

[I know, sounds like a bit of a hack copying the function and so on - maybe get the core team to move the core stylesheet functions into PageBase?]

Jonathan