[cloud import] Dark theme does not apply to uppy dashboard

[JammingBen]

The dark theme does not apply to the uppy dashboard which is used for the cloud import. We should use our color tokens here.

[dschmidt]

Especially the WebDav Formulars look pretty bad, we need to check whether we need to handle that in our upstream PR as well - somehow...

[dschmidt]

... oh and uppy has a dark mode as well, maybe we can just use that. We would need to tie it to our own dark mode switch then