xpi/content/css/close_icon_g_w7v2.css
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

#sidebar-close .toolbarbutton-icon,
*:not(#TabsToolbar):not(.tab-content) > .close-icon{
  list-style-image: url("chrome://classic_theme_restorer/content/images/close_w7v2.png") !important;
  -moz-image-region: rect(0, 16px, 16px, 0) !important;
}
#sidebar-close:hover .toolbarbutton-icon,
*:not(#TabsToolbar):not(.tab-content) > .close-icon:hover{
  -moz-image-region: rect(0, 32px, 16px, 16px) !important;
}
#sidebar-close:hover:active .toolbarbutton-icon,
*:not(#TabsToolbar):not(.tab-content) > .close-icon:hover:active {
  -moz-image-region: rect(0, 48px, 16px, 32px) !important;
}
@media (min-resolution: 1.1dppx) {
  #sidebar-close .toolbarbutton-icon,
  *:not(#TabsToolbar):not(.tab-content) > .close-icon {
	list-style-image: url("chrome://classic_theme_restorer/content/images/close_w7v2_2x.png") !important;
	-moz-image-region: rect(0, 32px, 32px, 0) !important;
  }
  #sidebar-close:hover  .toolbarbutton-icon,
  *:not(#TabsToolbar):not(.tab-content) > .close-icon:hover {
	-moz-image-region: rect(0, 64px, 32px, 32px) !important;
  }
  #sidebar-close:hover:active .toolbarbutton-icon,
  *:not(#TabsToolbar):not(.tab-content) > .close-icon:hover:active {
	-moz-image-region: rect(0, 96px, 32px, 64px) !important;
  }
}
#sidebar-close .toolbarbutton-icon,
*:not(#TabsToolbar):not(.tab-content) > .close-icon > .toolbarbutton-icon {
  width: 16px !important;
}
#sidebar-close {
  background: unset !important;
}