Dev Tools worked partially for me. I was able to create a new custom extension and template as described above from HTML5 default template, but there appear to be 2 major bugs:
1) Slide show feature does not work in the new extension template (instead of the "slide" class it infuses the "wide_block" id into the "sep" class div). The result is that the slide functionality does not work and the slides just show all on the page aligned to the left.
2) A number of blocks (including the slide blocks) show up as duplicates in the block manager list

Any hint on how to fix this?
Thx
Wolf