Many popular apps and browsers have introduced a dark mode. Essentially it toggles the background to black, adjusting the text and icons accordingly to work on a dark background.
As well as making colours pop and increasing contrast, there are practical benefits.
In a dark room, a bright white screen can be uncomfortable to use. A darker screen can be more relaxing and reduce eye strain.
Most people now use smartphones and tablets. Using a black background instead of white can improve battery life. Google calculated that, when used at 100% brightness, their YouTube app can save 60% of the screen energy when using dark mode compared to using a flat white background. With modern mobile devices using OLED over LCD for their screens, that benefit increases as more people upgrade.
In the top-right corner you should see a toggle to enable dark mode:
You should then see the interface change accordingly with the text changing to a lighter palette to work on the dark background:
Dark mode remains optional with the default setting being off since during the day, in a bright room, dark mode can actually be more uncomfortable with the text harder to read.