Yesterday Twitter.com in a tweet launched its Night Mode feature for desktops. This feature has been available a year now on IOS and Android phones.
— Twitter Support (@TwitterSupport) September 6, 2017
To Active 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.