Thanks very much! In the meantime I also listed out that the Item style cell for the SlideShow environment is also included  in the built-in Steel stylesheet and can be copied over to the local definitions and then edited. Your example of the editing the CellDingbat through the Option Inspector was though new to me - thanks!

Now knowing the solution I wonder why not all the different screen environment options are immediately inherited locally when one uses a template for a new slide show - it would be much simpler if one did not need to do this change manually. Anyone from Wolfram to comment? Of course in the continuation I can use my own modified slide to create further new ones, but still.