To change the theme, language, or time zone, go to the profile screen by clicking on your photo, name, and role in the sidebar.

By default, the color theme is set to automatic—it follows the system theme of your device. To change the theme, select one of the options in the corresponding section of the profile screen ①: automatic, light, or dark.
By default, the interface language is set to automatic—it follows the system language of your device. To change the language, select one of the available options in the corresponding section of the profile screen ②.
By default, the time zone is determined automatically. To change it, select the desired time zone in the corresponding section of the profile screen ③.