fringe-mode

fringe-mode is a variable defined in `fringe.el'.
Its value is

nil

Documentation:
Default appearance of fringes on all frames.
The Lisp value should be one of the following:
- nil, which means the default width (8 pixels).
- a cons cell (LEFT . RIGHT), where LEFT and RIGHT are
respectively the left and right fringe widths in pixels, or
nil (meaning the default width).
- a single integer, which specifies the pixel widths of both
fringes.
Note that the actual width may be rounded up to ensure that the
sum of the width of the left and right fringes is a multiple of
the frame's character width. However, a fringe width of 0 is
never rounded.

When setting this variable from Customize, the user can choose
from the mnemonic fringe mode names defined in `fringe-styles'.

When setting this variable in a Lisp program, call
`set-fringe-mode' afterward to make it take real effect.

To modify the appearance of the fringe in a specific frame, use
the interactive function `set-fringe-style'.

You can customize this variable.