Toshiba Satellite M840 unable to adjust screen brightness on Windows 8.1

Screen brightness becomes unavailable
after Toshiba Satellite M840 upgrading to Windows 8.1

Here is a warning for Toshiba Satellite M840 users. Do NOT upgrade to Windows 8.1 yet! Toshiba has not yet have proper drivers for the upgraded system. You will be unable to adjust the screen brightness after upgrade. All other functions (Fn+F1, F4-F12) have no problems; Fn-F2 and Fn-F3 for screen brightness adjusts fail.

I have yet to found a solution to it. Calling to Toshiba Support tells me I have to downgrade to Windows 8 (8.0) for now.