Instalei o gdebi e tentei reproduzir seu erro. Mas sem sucesso, como você pode ver na imagem. Você pode me dizer qual tema de canela você usou? Você está usando 2d fallback ou 3d ??
Essa é a seção de diálogo modal do meu tema canela:
/* ===================================================================
* Modal dialogs
* ===================================================================*/
.show-processes-dialog-subject,
.mount-question-dialog-subject,
.end-session-dialog-subject {
font-weight: bold;
color: #fff;
}
.modal-dialog {
border: 2px solid rgba(255,255,255,0.6);
background-gradient-direction: vertical;
background-gradient-start: rgba(32,32,32,1.0);
background-gradient-end: rgba(32,32,32,1.0);
box-shadow: 0px 0px 2px 1px rgba(0,0,0,0.5);
border-radius: 0;
color: #fff;
padding: 30px 42px;
}
.modal-dialog-button-box {
spacing: 21px;
padding-top: 50px;
}
.modal-dialog-button {
border-radius: 2px;
color: #fff;
margin-left: 10px;
margin-right: 10px;
padding: 4px 32px 5px;
border-width: 1px;
border-color: rgba(78,78,78,0.9);
background-gradient-direction: vertical;
background-gradient-start: rgba(58,58,58,1.0);
background-gradient-end: rgba(32,32,32,1.0);
box-shadow: 0px 0px 3px 1px rgba(0,0,0,0.5);
}
.modal-dialog-button:active,
.modal-dialog-button:pressed {
border-color: rgba(98,98,98,0.4);
background-gradient-direction: vertical;
background-gradient-start: rgba(28,28,28,1.0);
background-gradient-end: rgba(32,32,32,1.0);
box-shadow: inset 0px 1px 5px rgba(0,0,0,0.35);
}
.modal-dialog-button:active:hover,
.modal-dialog-button:pressed:hover {
border-color: rgba(98,98,98,0.4);
background-gradient-direction: vertical;
background-gradient-start: rgba(28,28,28,1.0);
background-gradient-end: rgba(32,32,32,1.0);
}
.modal-dialog-button:hover {
border-color: rgba(98,98,98,0.9);
background-gradient-direction: vertical;
background-gradient-start: rgba(68,68,68,1.0);
background-gradient-end: rgba(58,58,58,1.0);
}
.modal-dialog-button:disabled {
color: rgb(60,60,60);
}
.modal-dialog-button:focus {
box-shadow: 0px 0px 2px 1px rgba(117,145,169,0.9);
}