Is there a stylesheet property that can be set to control the extra level of indentation of a cell in its group (I don't want to specify absolute CellMargins)?
Carl suggests in the comments to use this in the stylesheet:
Cell[StyleData["Item"], CellMargins -> {{Inherited + 20, 10}, {10, 5}}]
which yields:
But this looks bad, I'd like them to be indented left-horizontally flush to their cell group's parent cell, plus a fixed about. Would look approximately like this (I used the ruler toolbar):


CellMargins -> {{Inherited + 10, 10}, {10, 5}}? – Carl Woll Mar 01 '17 at 23:15Inheritedwas clearer about use cases involving cell groups and in stylesheets. – user5601 Mar 02 '17 at 01:29