A perfect MacBookAir 6,2 should have backlight brightness controlled by designated keys on the Windows Keyboards, in our HP X360 and Envy 4-1117nr cases, they’re F2 & F3. Thanks to Rehabman tutoring me through the process on Tonymacx86.com, I got the brightness keys F2 & F3 working as in Windows on my Yosemite 10.10.4.