15Feb2013: Slacker has updated the interface for the player & site. I won't be updating this style... it would require actually looking the new player & site, which I just can't do. It's so... puke-a-licious... puke-tastic... yeesh.
18Feb2013: Here's some code you can use if you want, to make it tolerably less bright:

/* Slacker Radio (Feb2013) dimmer */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
@-moz-document url-prefix(chrome:), url-prefix(http) {
#main-window[stylish-url*="slacker.com"] #browser {background-color:#012!important;}
#main-window[stylish-url*="slacker.com"] #content {opacity:0.333!important;}
#main-window[stylish-url*="slacker.com"] #content:hover {opacity:0.666!important;}
}/* rule of thirds */

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If you want your active tab to use the old Slacker Radio gold color for its background (which is good if you setup a Firefox Portable instance just for Slacker Radio):

/* Fx active tab w/Slacker Radio gold bg */
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
tab.tabbrowser-tab[selected="true"]
{background-image:-moz-linear-gradient(bottom,#ad8819,#ded3b2)!important;
border-top-left-radius:9px!important;
border-top-right-radius:9px!important;}
.tabbrowser-tab[selected="true"] .tab-text
{color:#000!important;
font-weight:500!important;
font-family:Segoe UI!important;
text-shadow:none!important;}
/*e*/

(or you could just use my other Slacker style, http://freestyler.ws/style/125268/fx-header-like-slacker-radio-persona, which does a little more--and, in fact, using the styles together allows you to get [with a little extra code] nearly flawless continuity between the Firefox header and the slacker.com page to enable http://cdn.userstyles.org/style_screenshots/35239_additional_4399.png [oooh! ahhh!])