Broken User menu style for button

[markusguenther]

Description

Since version 5.2 the logout button of backend modules looks pretty ugly.
The button has a border and that leads to a look that does not match our flat design.

Steps to Reproduce

1. Log in to a neos newer or equal than version 5.2
2. Open a backend module like the media module or user management
3. Open the user menu in the upper right corner

Expected behavior

Logout button looks like the other buttons and is flat.

Actual behavior

Button has a border and looks therefore pretty ugly.

Affected Versions

Neos: 5.2