toolbars' & others backgrounds

Fonds et bas-fonds

in your userChrome.css
~/.mozilla/firefox/profile name/chrome/userChrome.css
(if it does not exist : create folder and file)

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* for a dark theme */
#navigator-toolbox toolbar#toolbar-menubar,
#navigator-toolbox toolbar#PersonalToolbar,
#navigator-toolbox toolbar#nav-bar,
#urlbar,
#urlbar *,
#searchbar .searchbar-textbox,
#searchbar .searchbar-engine-button,
statusbar,
statusbar *
{
-moz-appearance:none!important;
background:#121212!important;
}
/* color of text in urlbar, searchbar & statusbar */
#searchbar .searchbar-textbox,
#urlbar .textbox-input-box,
statusbar
{
color:#ffffff!important;
}

evidently, you can separate the code for toolbars, statusbar, etc
& you can only write for all toolbars

#navigator-toolbox toolbar

use gColor2 to find the hexacode of your colors
or see here for a basic color code, in this case, you can write i.e. (much more poetic)

statusbar
{
color:lightsalmon!important;
}