Twitter is currently rolling out the Night Mode feature for its Desktop Web App. The Night mode feature has been available to users on iOS, Android and Windows Phone for some time now. The Night Mode for its Web App has been in testing for some time now.
The Night Mode feature gives users a beautiful Tumblr like navy blue and fill up the moon icon with blue. To enable the Night Mode on Desktop user would need to click on the profile photo and select Night Mode, the last option with a moon icon. You can disable it by again clicking on the same option.
— Twitter Support (@TwitterSupport) September 6, 2017
The Night Mode feature replaces the bright white background with a sort of navy blue color. The latest Twitter update was announced on its Support channel.