Notifications icons aren't showing properly for Color Themes "Darkly " and "Superhero"

Preconditions: Have some notifications in the notification list.

Steps to reproduce:

1. As Admin click on Drawer > Administer Wiki > Look & Feel > Themes
2. Under Color Theme select from the drop down list "Darkly"
3. Click on the Save button
4. Click on the Watchlist bell from the top right corner
5. Observe the UI
6. Select the "Superhero" theme
7. Click on the Save button
8. Click on the Watchlist bell from the top right corner
9. Observe the UI

Expected results: The UI looks good.

Actual results: Some icons don't appear for this 2 themes.