There is a 'funny' dependency between
Let's quote ref/CellFrameMargins details and options:
CellFrameMargins defines the margins for the box used to draw the background color of a cell. The margins will be used to define the extent of the background color even if there is no
Background->None is set, then the margins are only applied if a
CellFrame is present. If the cell frame isn't drawn on all sides, then the margin will only be applied to the sides with a frame setting.
Which translates to:
no background and no frame ==> no margins
no background and a frame ==> margins respected for sides that have frame
background =!= none ==> margins respected
Your initial case is (1.) and you need to know that
"Item" style has
CellFrameMargins (which are ignored now) but has no
Once you add
Background we have the 3rd case.
Having those rules laid out we can come up with a solution. That is the only way is to make
CellFrameMargins -> None. Why aren't they
None by default for
Item? Or why is it so convoluted? Ask WRI Support.
You can do this via stylesheet for
"Item" style or you can patch a specific notebook with:
SetOptions[#, CellFrameMargins -> None] & /@ Cells[CellStyle -> "Item"]