How Do I Get The Screen On My Mac To Stop Dimming When Watching Videos?
There are a couple options. First, you can go to System Preferences > Energy Saver and drag the slider bar for "Display sleep" so that your display never goes to sleep. We don't recommend this option since it will shorten the life of your display.
The second option is to use an app like Caffeine on your Mac. Caffeine is a free download from the Mac App Store that adds a small coffee cup icon to the right side of the menu bar. Any time you want to prevent your display from dimming, just click the coffee cup. It toggles between appearing empty (dimming allowed) and full of coffee (no dimming/sleeping).