Enhancement: Align dark mode colors with given design

The colors in the dark mode have been aligned with the given design. This improves the overall look and readability.

https://github.com/owncloud/web/issues/7373
https://github.com/owncloud/web/issues/7353
https://github.com/owncloud/web/issues/7738
https://github.com/owncloud/web/pull/7862
