First introduced with macOS Mojave, the Dynamic Desktop theme is so-called because the image shown on the desktop changes throughout the day. If you go to System Preferences>Desktop & Screen Saver>Desktop and select the ‘Dynamic’ theme for one of the stock Apple desktop images, the lighting in the image will go from dark, to light, to dark again to reflect the time of day. As you will discover here, it is possible to use one of your own images for a Dynamic Desktop theme. You just need to either have multiple versions of the same picture taken at different times of day (unlikely), or cheat and adjust the exposure of a single image to give the illusion of it getting darker.
Dynamic Desktop is a cool feature of macOS, but did you know that you can create your own?