Hi All Dent,
# ***Backlight Cable/Connector:*** I have had this same problem happen to me on an older MacBook and the issue was found in the cable that controlled the backlight. If the cable is broken, not plugged in, or just not plugged in the whole way, it can cause this problem. It can be an easy cable to miss when you put a laptop back together.
# ***Backlight:*** If the cable has no flaw to it, it could be that the backlight itself is dead. This would be replacing either the whole LCD panel or the whole display assembly if necessary.
# ***Ambient Light Sensor:*** If your Chromebook happens to have an ambient light sensor on it, the problem could reside there. If something is blocking this sensor, it could be making the display dim and not get to its full brightness. These sensors can usually be manually overridden by changing the brightness on your computer but I don't know exactly because it could be different for your computer.
# ***Other:*** If none of these fix your issue, the problem could be damage to the logic board or any components on it.
Thanks, Derek Thomas