Teste bitte mal:
CSS
/***********3 Buttons rechts oben******************/
#main-window .titlebar-min:hover{
background:#cccccc!important;
}
#main-window .titlebar-max:hover{
background:#cccccc!important;
}
#main-window .titlebar-close:hover{
background:#ee5500!important;
}
#main-window .titlebar-min,
#main-window .titlebar-max,
#main-window .titlebar-close {
border: 1px solid grey !important;
margin-top: -4px !important;
margin-bottom: 1px !important;
border-radius: 7px !important;
}