"X" in top right corner of Settings page doesn't have an on hover effect

[jasonblais]

I confirm (by marking "x" in the [ ] below):

• This is not a troubleshooting question. Troubleshooting questions go here: http://www.mattermost.org/troubleshoot/.
• This doesn't reproduce on web browsers (such as in Chrome). If it does, issue reports go to the Mattermost Server repository.
• I have read contributing guidelines.

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Summary
"X" in top right corner of Settings page doesn't have an on hover effect

Steps to reproduce

Windows 10; Mattermost Server 3.6; Mattermost Desktop 3.5

1. Go to File > Settings
2. Hover over the "X" in top right corner

Expected behavior
On hover effect following Mattermost UX guidelines.

This can be similar to the on hover effect of the "+" icon in desktop server tab bar.

Observed behavior

No on hover effect

[wvds]

@jasonblais I've made a small PR where the color of the icon changes to the same color as the "add server" icon. In my opinion that is the nicest solution :)