Yea I know what you mean, it drives straight into darkness if both sensors are equal.
There are two solutions:
The first (and easiest way) is to reduce the threshold. For example, threshold = 1. The lower the threshold, the less likely the robot will go straight.
In the code you have to change it so it does something like:
If left_sensor + right_sensor = really_small_number //robot is in darkness
then do something about it (like do a 180 to face the other direction);
You will have to tweak really_small_number until you are satisfied with it. Try 30 and see what happens.
When you get USB/serial added to your robot, it will be able to output the sensor data on your screen so you dont have to guess it. Im working on that tutorial . . . You can also use your LED to output information, for example, blink once if the sensor is 10, blink twice if the sensor is 20, three times for 30, etc.