Hello, I hope the following is of assistance.
The footwell lighting on the new model Focus are LEDs therefore if one is dimmer than the other it's 95% that the LED is failing. All the LEDs are powered by the body control module and are coded so you can't just swap one over to try it as if you swap the LED positions the ones in the incorrect place won't work at all. It's unlikely to be a body control module fault as the output from this is the same for all the interior LEDs.
As the car is a 2013 model this will be repaired free of charge under the manufacturers 60000 mile/3 year warranty by your local dealer.
I hope this helps but if I can assist further let me know and I'll reply as soon as I'm able to.