index.css

.toolbar-button-with-text:before {
	display: block;
	top: 3px;
	bottom: 3px;
	left: 3px;
	right: 3px;
	position: absolute;
	text-align: center;
	font-size: 20px;
	font-weight: 500;
	line-height: 30px;
	border-radius: 4px;
}

.toolbar-button-with-text:hover:before,
.toolbar-button-with-text:focus:before {
	box-shadow: inset 0 0 0 1px #555d66, inset 0 0 0 2px #fff;
	color: #555d66;
}

.toolbar-button-with-text.is-active:before {
	background: #555d66;
	color: #fff;
}

.toolbar-button__advanced-sup:before {
	content: 'x²';
}

.toolbar-button__advanced-sub:before {
	content: 'x₂';
}