Since macOS Mojave, a wallpaper can change by itself, either over the course of the day or when your Mac switches between light and dark mode. Behind both is the same file format, and you can make these files yourself.
What is inside the file
A dynamic wallpaper is a HEIC file, the image format Apple also uses for iPhone photos. Unlike a JPG, a HEIC file can hold several images at once. macOS reads an extra piece of metadata that tells it which image belongs to which situation.
There are two kinds:
- Light/dark wallpapers contain two images, one tagged for the light and one for the dark appearance. This is the kind that switches with dark mode.
- Time and sun wallpapers contain many images, each tagged with a time of day or a position of the sun. These change gradually during the day, like Apple's desert and landscape wallpapers.
For most people the light/dark kind is the useful one: two versions of the same design, one for the day and one for the evening.
What makes a good pair
The two images should clearly be the same picture. Keep the layout and the text in the same place and change only the colors and the brightness. If the design jumps around, the switch feels like a glitch instead of a transition. The article on choosing colors for light and dark has tips for pairs that work.
Three ways to make one
1. In the browser, free. The Notchwerk generator lets you pick a template, write your own text and choose light and dark colors. It renders both images at your Mac's native resolution and packs them into a HEIC file with the right tags. Nothing to install.
2. With a Mac app. If you already have two images of your own, an app like Dynaper from the Mac App Store can combine them into a light/dark wallpaper. It also handles the time-of-day kind.
3. On the command line. The open source tool wallpapper builds dynamic HEIC files from a set of images and a small JSON file that describes them. Useful if you want to automate the process.
Setting it up
Save the file somewhere permanent, right-click it in Finder and set it as your desktop picture. Then choose the automatic option in System Settings → Wallpaper. The step-by-step guide covers the details, and these five fixes help if it does not change.