I use mobile devices like my phone and tablet in dimly lit rooms a lot. The lowest brightness settings usually works well for me, except when I have a migraine and am lying down in a dark room and at night. I read, listen to audio books or play games and even the dimmest settings is still too bright to be comfortable.
Screen Filter is a free Android app that is easy to use and applies a shade that works as a dimmer to further reduce the screen brightness. It is a simple app. Hit the shortcut button once and it applies the shade and hit it again to remove it. The level it dims to can be set and saved.
It is the blue light coming from the screen that causes eye strain. There isn’t an App that I know of that lets you lower the colour temperature unless you root or jailbreak your phone. Turning down the brightness further is the next best thing. I mostly use my phone for very short periods when I have a migraine, like finding the right spot in an audio book, checking the time or my email and a dimmer works perfectly to lower the light coming from the screen to a tolerable level.