set-window-margins

set-window-margins is a built-in function in `C source code'.

(set-window-margins WINDOW LEFT-WIDTH &optional RIGHT-WIDTH)

Set width of marginal areas of window WINDOW.
WINDOW must be a live window and defaults to the selected one.

Second arg LEFT-WIDTH specifies the number of character cells to
reserve for the left marginal area. Optional third arg RIGHT-WIDTH
does the same for the right marginal area. A nil width parameter
means no margin.

Return t if any margin was actually changed and nil otherwise.