If you want to automate your screen going from light mode to dark mode on your Mojave updated mac, use NightOwl.

Upon downloading NightOwl, copying it to your applications folder, and running it, NightOwl will add an owl icon to your menu bar.

 Clicking on this will cause a drop down menu to come down. 

The app will ask you for permission to change settings.  drop-down is so that if can change the mode itself.

Here you can click on Light or Dark to change your current mode.

By clicking Sunrise/Sunset, your desktop will automatically change at that time of day.  The app will ask your permission to use your location (just to get the time of sunrise and sunset of your current location).

Or you can schedule the change yourself.

NightOwl is a free app, but you might want to help the developer out and contribute some money.  This things don’t happen for free.  

I’d still put money on Apple adding this feature in a future update, but for now.

I found this app through the Six Colors website.