toolbar button highlight color

I’m trying to figure out how the toolbar button highlight color is set. I’ve examined all the various components with DOM Inspector + Context Inspector and am coming up with nada. I’m beginning to believe that the critical property is defined !important in the base code; that would preclude overloading / over-riding the property, or applying alternate attrbitute to said property.

I have a style with the following namespace:

@namespace url(Mozilla XML Namespace);
@-moz-document url(‘chrome://browser/content/browser.xul’)

I dunno if I need both (or only the latter). In any case, I can’t figure out why this doesn’t work (emphasis mine):

toolbarbutton.bookmark-item:hover {

    font-weight:bold !important;
    border-style:none !important;
    -moz-appearance:none !important;
    /* [i][b]background-image: [/b][/i]url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAEAAAAAoCAIAAADBrGu+AAAACXBIWXMAAAsTAAALEwEAmpwYAAAAR0lEQVR4nO3PQQ0AIBDAMMC/50MEj4ZkVbDtWX87OuBVA1oDWgNaA1oDWgNaA1oDWgNaA1oDWgNaA1oDWgNaA1oDWgNaA9oF3z8BT6BPhC0AAAAASUVORK5CYII=) !important; */
    [i][b]background-color: #FF0000 !important;[/b][/i] 

}

The comment block contains code that references a red square PNG image. I tried that thinking the toolbar button highlight is a background image. Obviously I’d remark out the background-color attribute. Doesn’t work either way. Nor does it work if I use the ‘color’ attribute.

What does happen is when mousing over the Bookmark Toolbar menuitems, toolbarbutton.bookmark-item .toolbarbutton-text becomes bold.

I tried changing the system SELECT color to red, and that didn’t work either. System SELECT became red, but mouseover toolbar buttons still resulted in blue highlight.

I’ve been struggling with this for several days now. None of the proffered solutions on the interwebs are efficacious.

I’ve determined that the toolbar-button is extended class of button. The former inherits properties and attributes from the latter. For the former, the default XUL and CSS define the button background as transparent. Moreover, class button implements Mozilla CSS extension moz-gradient and 3DFace.

I’ve not pinned down how to over-ride that yet. But it can be done using Stylish add-on to over-ride the proper CSS selector.