You have this .k-state-disabled class in kendo.css but it's not used in scripts. That's why I forced to copy all .k-state-disabled class styles to new class like

.k-button:disabled, .k-button:disabled:hover, .k-button:disabled:active {
background-color: #E3E3E3;
border-color: #C5C5C5;
color: #9F9E9E;
opacity: 0.7;
cursor: default;
outline: 0 none;
}

(this is my temporary solution)