Button icon hover

I'm trying to make it so that if a user hovers over a button, it changes the icon's image. I've tried editing the CSS by adding the following rule, but it didn't work:

.cke_button:hover .cke_button__bold_icon {
    background: url(icons_hover.png) no-repeat 0 -24px !important;
}

How can I change the icon image on hover?

Thanks.