How to adjust LEDs brightness with photoresistor?
Find a micro-controller with A2D and PWM features - one of the PIC-12 series should do.

wire up the LDR in a voltage divider configuration to the A2D input

wire up the LED to the PWM output

Create a power supply for the micro, using the LM317 or similar to reduce the 12V to the PIC supply voltage (5V or 3.3V)

Write a program to read the LDR resistance as a voltage, and output an LED brightness as a PWM percentage

Program your micro, test and debug.