To enable Night Mode on desktop, click on your profile photo, then select Night Mode, which is the last option and has a moon icon. Activating Night Mode turns Twitter’s usual white screens and cheerful light blue into a beautiful Tumblr-like navy blue and fills up the moon icon with blue. To disable Night Mode, just click again to deselect.
Since I’ve customized my hyperlinks to show up as purple and set my header image as a purple cascade of lines, Night Mode is especially aesthetically pleasing. It loses Twitter’s bright and cheerful UI and gives everything an early Halloween vibe.