A System theme that follows your OS
A new System theme follows your OS light or dark setting. It is the new default, and accounts on Default move to it; any other theme stays.
System follows your OS. The new System theme shows the Dark theme when your operating system is set to dark, and the light Default look otherwise. It checks once per page load, so a chart never changes colour while you read it. Pick it, or any of the other 13 themes, under appearance on your account page.
Default moves to System. System is now the theme a new account starts on, unless you picked a mode on the website first. Accounts that were on Default have moved to it, so if your OS is dark, the app is dark now too. Pick Default again to keep the light look. Any other theme you chose stays as it was.
Native controls match. Dark themes now tell the browser they are dark, so scrollbars, date pickers and other native controls render dark instead of white. The Dark theme draws its accent buttons in white with dark text instead of pink. The legal pages on the app follow your theme too.
Pick before you sign up. The website has a light and dark switch in its header, or in its menu on a phone. Use it before you sign up and your new account starts in that mode: Default for light, Dark for dark. After that the switch changes only the website, never your account's theme.
Docs: the web UI.