MediaWiki:Common.css

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.collapseButton {		/* 'show'/'hide' buttons created dynamically by the        */
	float: left;		/* CollapsibleTables JavaScript in [[MediaWiki:Common.js]] */
	font-weight: 800;	/* are styled here so they can be customised.              */
	text-align: right;
	width: auto;
}
<!-- No [History] tab for anonymous users -->
<?php global $wgUser; if( $wgUser->isAnon() ) { ?>
      <style type="text/css">
        #ca-viewsource { display: none !important; }
      </style> 
    <?php } ?>