How Do I Get The Screen On My Mac To Stop Dimming When Watching Videos?
There are several options available to stop a Mac screen from automatically dimming due to inactivity. First, you can open the System Preferences app and navigate to the Energy Saver panel. Drag the slider bar for "Turn display off after" so that the display never goes to sleep. As warning, choosing this option may shorten the life of your display.
The second option is to use a Mac app like Amphetamine to temporarily adjust the display settings. Amphetamine is a free download from the Mac App Store that adds an icon to your Mac menu bar. Any time you want to prevent your display from dimming, just click the menu bar icon.