Override ui-icon-closethick on the dialog?

Override ui-icon-closethick on the dialog?


Ok, I'm fairly familiar with CSS, but I'm no expert. Can someone
please tell me how to create a custom button JUST for the dialog? I've
been able to override other elements ok, but ui-icon-closethick just
wont work. I simply want to replace that boring grey "X" with a nice
red glossy button that matches the rest of my site. Is that even
possible? I was hoping it would be something as simple as this:
.ui-icon-closethick {
    height: 18px;
    width:24px;
    background-image: url(/platform/images/small_close_button.gif);
    background-repeat:no-repeat;
    display:block;
    cursor:pointer;
}
Nope! That don't work at all. I was able to override the title bar
easy, but not this darn button!
Any help would be greatly appreciated!