Honeywell WiFi thermostat install
Yes, you are right on track here

Simply connecting the light blue wire to the C terminal on your furnace control board (with the furnace turned off at the service switch or breaker, of course) will get you a C wire for your new thermostat.  You are also correct that the cable with the yellow and dark blue wires connected in it goes off to your air conditioner (outdoor unit); it is what carries the signal to the air conditioner that calls for it to run the compressor.