Edit in JSFiddle

$(function() {
    $("#btnClose").button({
        icons: {
            primary: "ui-icon-closethick"
        }
    });
    
    $("#btnClose1").button({
        icons: {
            primary: "ui-icon-closethick"
        }, text: false
    });
});

<p>Button looking odd,</p><button id="btnClose" class="Close"></button>
<br/><br/>
<p>Button looking good,</p><button id="btnClose1" class="Close">Close </button>