.mce-i-ucalc-ico:before {
    content: "";
    background: url(https://ucalc.pro/favicon.png) no-repeat center;
    background-size: 16px;
    display: block;
    width: 20px;
    height: 20px;

}
#qt_content_button_uCalc {
    content: "";
    background: url(https://ucalc.pro/favicon.png) no-repeat center;
    background-size: 16px;
    width: 26px;
    height: 26px;

}