Minor css fix [icons show unexpected behavior on hover]

Steps to reproduce the bug:

**Pre requisite**: Sign in as root

1. Create a new space.
2. Go to the space homepage, Click the "Administration" dropdown.
3. Hover on the items thus displayed by the dropdown.

**Expected outcome**: The items are displayed with proper css properties.

**Outcome**: The items are being displayed with a "icon-filter" as background.

**Fix** : changed the css hover properties in screen.css