If the backlight is off, or certain keys do not respond, on your Slim X3 Bluetooth Backlit Keyboard (ST-BTSX3M, ST-BTSX3S), check these points.

How to fix the backlight or keys on the Slim X3

1. Charge the keyboard, and make sure it is paired and connected. The backlight on the Slim X3 only works when the keyboard has an active connection to a paired device, so it will not turn on if the keyboard is unpaired or off.
2. Press the backlight key to cycle through the brightness levels. The Slim X3 has several backlight brightness levels.
3. If the number keys do not type numbers on Windows, press the Clear key, which acts as the Num Lock key on the Slim X3.
4. Remember the Slim X3 uses an Apple key layout, so on Windows some keys map differently. For example, the Clear key works as Num Lock, so check a key's Windows function if it does not do what you expect.
5. Connect the keyboard in wired mode with the included USB-C cable, by pressing Fn and Eject, to confirm whether a key fault is hardware or connection related.