The backlight brightness of an Lenovo E145 laptop with a Radeon HD 8240 graphics controller can not be adjusted on Debian. Neither the Fn-F7/Fn-F8 keys are working nor any other interface.
Force the use of the kernel Radeon driver by installing the (non-free) Radeon firmware.
There are several drivers that may be used by the Linux kernel and the X Windows System. The trick is forcing the correct one, as not all drivers support the backlight.