A couple years after the GBA SP (AGS-001) a new revision of it, the AGS-101, came to market.
That revision used a backlit screen and the toggle button allowed you to switch between a dim and bright backlight mode.
If my memory serves right games on that revision looked like they would on the original Nintendo DS.
Obviously battery life took a significant hit compared to playing on the AGS-001 with the frontlight disabled.