The Brightness option in the Super Smash Bros. series was first introduced in Super Smash Bros. Ultimate, under the Options menu. This allows you to adjust the brightness of the screen.
In Super Smash Bros. Ultimate
In Ultimate, the player can create different brightness settings for whether the Nintendo Switch is in TV-Mode or Handheld-Mode, automatically changing if the system is docked or not. A slider can be found to adjust the brightness, at a default brightness of 50. A still image of Mario and Bowser fighting on Battlefield and a greyscale spectrum of the Super Smash Bros. logo is used to easily compare different brightness settings. Pressing X will reset the brightness slider to 50, and pressing Y will fullscreen the still image for a better comparison.
Gallery
The screenshot of a fight used in Super Smash Bros. Ultimate
See also