First, you'll need to head over to your iPhone's Settings. Once there, navigate to General > Accessibility > Zoom, and toggle Zoom on.
Next, you'll need to take three fingers and triple tap anywhere on the screen, like so.
Next, tap Full Screen Zoom and then drag a finger on the slider to zoom all the out.
You'll then need to tap Choose Filter and then tap Low Light, which will darken your screen.
You've now set up your iPhone so when you have Zoomenabled it acts as a dimmer switch, but now we need to make a shortcut so it's lightning fast to darken your phone.
To do this, go back to Accessibility and scroll to bottom and tap Accessibility Shortcut. Now, just tap Zoom and you're all set!
Now, when you triple tap your iPhone's home button, it will act as a dimmer switch and darken your phone. Remember, if you find your screen is too dark to see anything, just triple click the home button to turn the dimmer off.
If you have any questions, Snazzy Labs offers a full video tutorial below.