The macOS Mojave 10.14 brings a long-awaited systemwide Dark Mode, with Mojave users able to choose between a light theme or the new dark theme. Dark Mode of Mojave is accompanied by Dynamic Desktops, aka wallpapers that subtly change throughout the day.
You can download and install Apple MacOS Mojave 10.14 on your Mac.
Here is the macOS Mojave installation guide by SYSNETTECH Solutions:
macOS Mojave 10.14 Installation Guide