As someone who is not a Mathematica expert, I find the stylesheet editor deceptively easy looking but relatively useless to a nubie for formatting a slideshow. Upon opening the editor one can choose a style and then format it using the front end capabilities. The result shows the style name in the new style. One can then choose another style and format it as well. When done, if desired one can install the edited stylesheet for use with other slideshows. If this were all that was necessary then I would be quite content with the editor.

But this only formats the working environment and not the slideshow environment. As far as I am able to tell, to define the slideshow environment styles one must first select each cell in the stylesheet, show the code behind the formatting cell (not an obvious step to a newbie such as me), and edit the cell from, for example,

Cell[StyleData["Section"] ...

to

Cell[StyleData["Section", "SlideShow"] ...

I may be missing an editing command that would globally apply formatting to the slideshow view, but even if there is such a command the default format editing for slideshows should apply to the slideshow view in preference to the working view.