tool-bar-button-margin

tool-bar-button-margin is a variable defined in `C source code'.
Its value is

4

Documentation:
Margin around tool-bar buttons in pixels.
If an integer, use that for both horizontal and vertical margins.
Otherwise, value should be a pair of integers `(HORZ . VERT)' with
HORZ specifying the horizontal margin, and VERT specifying the
vertical margin.