Customize Firefox App Menu Button’s Colors

#main-window[privatebrowsingmode="temporary"] #appmenu-button{
background: black !important;
border: 1px solid white !important;
}
or

#appmenu-button{
background-image:-moz-linear-gradient(rgba(200,200,200,1), rgba(200,200,200,1)) !important;
color:black!important;
font-weight: 900!important;
font-family:"Segoe UI"!important;
text-shadow: 0px 0px 5px white, 0px 0px 5px white, 0px 0px 5px white!important;
}

#main-window[privatebrowsingmode="temporary"] #appmenu-button{
background-image:-moz-linear-gradient(rgba(115,115,115,1), rgba(115,115,115,1)) !important;
color:black!important;
font-weight: 900!important;
font-family:"Segoe UI"!important;
text-shadow: 0px 0px 5px white, 0px 0px 5px white, 0px 0px 5px white!important;
}