Fix EZP-20878: Images inside CSS files fail to load in non vhost setup

JIRA issue: https://jira.ez.no/browse/EZP-20878

# Description

The path used for images in the CSS (less files) is not compatible with eZ Publish being installed in a sub-directory. This patch fixes this issue.
While at it, I also discovered that the files CSS and less were not in sync and that a less file contains a syntax error.

# Tests

manual tests

Side note: As mentioned in the https://github.com/ezsystems/ezdemo/pull/17, please please always make sure the CSS files are in sync the less files and don't sync CSS "blindly" from ezdemo but generate them with the less compiler from the local less files.