ZOOM   IN     Windows : Ctrl ➕ ("control" key and "➕" plus key) /
                           Mac :       ⌘  ➕ ("Command" key and "➕" plus key)

ZOOM OUT   Windows : Ctrl ➖ ("control" key and "➖" minus key) /
                           Mac :       ⌘  ➖ ("Command" key and "➖" minus key)

For Windows, please visit this page : f.lux for Windows
For iPhone, iPad, and iPod touch, please visit this page : f.lux for iOS (available for jailbroken iOS devices only)
For Android, please visit this page : f.lux for Android (most rooted Lollipop and Marshmallow devices (5.x and 6.x) will work) or use this free Android App : Blue Light Filter - Night Mode
For Apple computers, please visit this page : f.lux for Mac