Specifically I want different versions of 'Item1Numbered'. For example one with a frame and another with a background colour. How is that done?
You need to add
StyleDefinitions -> StyleData["ParentStyle"] as an option to the
StyleData of your new style (in the stylesheet), where
"ParentStyle" is whatever you want to inherit from.
In your case, go to
Format > Edit Stylesheet and add the following styles:
Cell[ StyleData["MyItemStyle1", StyleDefinitions -> StyleData["ItemNumbered"]], CellFrame -> True ] Cell[ StyleData["MyItemStyle2", StyleDefinitions -> StyleData["ItemNumbered"]], Background -> GrayLevel[0.8] ]