Choosing the theme and the language
Both choices are at the foot of every page, and both are remembered, so you make them once.
The theme
Theme offers Light, Dark and System. System, which is what you have until you choose, follows your phone or computer: the site is dark when it is, and light when it is not. Choose Light or Dark to have the site that way whatever your device does.
The language
Until you choose, the site speaks the first language your browser says you read that the site has. Where it has none of them it speaks English.
To choose another, pick it from the Language list and press Change. The page comes back in that language, and so does every page after it, on this visit and the next. The list opens on the language you are reading in.
A page of this help that has not yet been put into your language is shown in English, rather than not shown.
If a choice is forgotten
The site remembers each choice in a cookie, so a browser that clears its cookies, or is set not to keep them, forgets and asks you to choose again. Scripting and cookies says what the site keeps.
Next: Scripting and cookies.