Steps to reproduce

1. On a deployment which is configured to use dark theme by default (i.e. config.json has "default_theme": "dark". Equivalently, config_json has no setting for default_theme, but the system theme is dark).
2. Log out (or use incognito window).
3. Close tab.
4. Open a new tab, at <client uri>/#/user/@ (yes it's obscure). Note this doesn't happen if you just edit the URL from the login screen.
5. Observe a light-themed login window. Apparently that was deliberate (Add an override for the theme matrix-org/matrix-react-sdk#4014), though it seems an awful kludge
6. Log in
7. Complete or skip verification

Outcome

What did you expect?

Dark theme

What happened instead?

Light theme