PIC16F676 - Is it possible to add a dimmer circuit to the seven segment displays
You need to do two separate things:


Measure the light level. This should be done with a light dependent resistor and an analog input of the PIC. 
Adjust the brightness of the display. The simplest way I can think of is to reduce the duty cycle, the fraction of the time that the LEDs are on.
You are already be switching quickly between the separate digits, so it should be a simple matter to turn them each on for an even shorter period of time. This will make them appear less bright.


Then you can link the two with a formula or algorithm, which you can develop by experiment.

A note on multiplexing:
You can generally get away with flickering an LED at perhaps 50 or 100 Hz, if it's for a normal range of brightness. But for very short duty cycles (and you will need to go very short to get a dim display, perhaps 1/256), even 100 Hz starts to look strange, the lights leave dotted streaks as you look around the room. So aim for something more like 1 kHz, to be sure it looks smooth even at low brightness.