View file wordpress/wp-includes/js/tinymce/plugins/spellchecker/css/spellchecker.css

.mceMsgBox {
	border: 1px solid gray;
	padding: 8px;
}

.mceMsgBox span {
	vertical-align: top;
	color: #555555;
}

/* Misc */

.mceBlockBox {
	display: none;
	position: absolute;
	left: 0;
	top: 0;
	z-index: 100;
	filter:progid:DXImageTransform.Microsoft.Alpha(style=0, opacity=60);
	-moz-opacity:0.6;
	opacity: 0.6;
	background-color: white;
}

.mceMsgBox {
	display: none;
	z-index: 101;
	position: absolute;
	left: 0;
	top: 0;
	font-family: Arial, Verdana, Tahoma, Helvetica;
	font-weight: bold;
	font-size: 11px;
	background-color: #FFF;
}