I have used this code in my init call for tinymce to point at my main css file:
/* toolbar */
.mce-toolbar-grp {
background-color: #000 !important; /* uses !important or override .mce-panel background-color/image */
background-image: none !important;
}
/* text color */
#tinymce {
color: #dd9900;
}
/* button text color */
.mce-ico {
color: #dd9900;
}
/* button background color */
.mce-btn button {
background-color: #000000;
}