<p>I would like to allow the logged user to edit MediaWiki/Common.css without adding them to the sysop group.</p>

<p>I understand that this will allow user to change it to harful ways but it is a closed wiki so that is not a problem.</p>

<p>Any solution is acceptable even changing php code :)</p>

Create a new group, add give it "editinterface" privilege. In LocalSettings.php it's done like this:
```
```
$wgGroupPermissions['mynewgroup']['editinterface'] = true;
```
```
Then add the user to you new group.
Or if you want to give that right to all logged-in users, do it like this:
```
```
$wgGroupPermissions['user']['editinterface'] = true;
// user is the default group for all logged-in users
```
```
For details see
MediaWiki manual
.