tool-bar-border

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

internal-border-width

Documentation:
Border below tool-bar in pixels.
If an integer, use it as the height of the border.
If it is one of `internal-border-width' or `border-width', use the
value of the corresponding frame parameter.
Otherwise, no border is added below the tool-bar.