To work with Stylesheets proficiently you need to be able to edit Style
Cell code directly, not only make changes via the menus. While in most cases you can perform the same actions via the Option Inspector there is always a layer of abstraction between you and the code.
After opening the editor via Format > Edit Stylesheet ... you should see something like:
If the Style you wish to modify is not present you need to first create a Cell for it. This is done most conveniently using the Choose a style dropdown or by entering the exact Style name without quotation marks in the Enter a style name : field and pressing Return.
Once you have a Style Cell like this:
you can click on it and use Cell > Show Expression (Shift+Ctrl+E in Windows) to reveal:
It is that Cell data that you need to edit to take full control of the Style. Options for the Style are given as options for
Cell. So for example editing it to read:
CellFrame -> 0
will disable the Cell frame present on the Section style of the StandardReport base which we are modifying.