There are two options to switch between light and dark mode. The easiest option is by clicking the small lamp icon in the upper right hand corner of the page.
This icon is always visible regardless of the page that you have open. If the lampshade is empty, clicking on it will switch to dark mode. The change is immediate and will apply to all pages. It is also persistent and the dark mode option will stay activated even if you log out and log back in.
If the lampshade is full, clicking on it will activate light mode. Just as with changing to dark mode, the change is immediate, site wide, and persistent.
The other option for switching between light and dark mode is found on the Appearance page. This can be reached by clicking on the account name in the upper right hand corner of the screen, right next to the lamp icon, and selecting Appearance from the drop down menu.
On this page the third option down is the switch for light and dark mode. Clicking on the option that you wish will switch between modes.