una/template/css/auth.css

.sys-auth .bx-btn {
    float: none;
    width: 100%;
    text-align: center;
    text-transform: uppercase;
    font-weight: bold;
    white-space: nowrap;
}

.sys-auth-compact {
	position: relative;
	display: inline-block;
}

.sys-auth-compact .bx-btn {
	width: auto;
}