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

By default, the color theme is determined automatically. To change it, click “Theme” ①, select the desired option in the window that opens—“Automatic”, “Light”, or “Dark”—and click “Save”.

By default, the interface language is determined automatically. To change it, click “Language” ②, select the desired option in the window that opens, and click “Save”.
By default, the time zone is determined automatically. To change it, click “Time Zone” ③, select the desired option in the window that opens, and click “Save”.