Contrast issues in Dashboard page and notifications list when using dark themes set at page tree level

Preconditions: The Admin user has notifications activated and some events in the list

Steps to reproduce:

1. Log in as Admin
2. Activate notifications
3. Access the Dashboard page
4. Click on More Actions > Administer page
5. Select Look & Feel > Themes
6. Select and apply Darkly / Slate / Superhero
7. Access the Dashboard page

Expected results: The colors on the UI have proper contrast so that the UI is clear and easy to use by the user. All content is shown and readable.

Actual results: The colors used in the Activity Stream and in the Notifications list don't have the proper contrast (foreground / background) so the text is unreadable and the icons are not visible.