Hello yginnab,
We understand that you're looking for options outside of Control Center to adjust the backlight for your MacBook Air keyboard. Looking over the link below, there are 3 options only listed, and it doesn't mention adding additional keys to control this feature.
Turn your Mac keyboard’s backlighting on or off - Apple Support
Manually adjust keyboard backlighting or turn it off
Do one of the following, depending on your Mac model:
- On the keyboard, press the increase brightness key or the decrease brightness key . To turn backlighting off, keep pressing .
- In Control Center, click Keyboard Brightness, then drag the slider.
- In the Touch Bar, expand the Control Strip, then tap the increase brightness button or the decrease brightness button . To turn backlighting off, touch and hold .
Since additional options aren't provided, you can submit feedback to Apple regarding this feature. Please visit Product Feedback to provide your comments and suggestions.
Best regards.