Appearance and language
Choose a light or dark console, and the language the console's screens are shown in.
Who can do this: everyone, for themselves.
Open Settings, choose the You tab, then Appearance & language.
Theme
Under Theme ("Follow your system setting, or pin the console to light or dark."), pick Light, Dark or System. The change applies straight away.
The theme is remembered in this browser only. It is not shared with your teammates, and another browser or device keeps its own choice.
Console language
Pick a Console language: "The language every screen in this console is shown in. Your projects, their source strings and their translations are not affected." The list shows the languages the console is available in, each written in its own language.
The console language is saved to your login, so it follows you to any browser you sign in from. It changes the console only, not the languages of the projects you translate.