This will override all cancel button's styles in your app.
Include the following in your CSS and include your CSS after sencha's CSS:
/*Cancel Button styles overriding*/
.x-toolbar-dark .x-button, .x-toolbar-dark .x-button.x-button-back::after, .x-toolbar-dark .x-button.x-button-forward::after, .x-toolbar .x-toolbar-dark .x-button, .x-toolbar .x-toolbar-dark .x-button.x-button-back::after, .x-toolbar .x-toolbar-dark .x-button.x-button-forward::after, .x-toolbar-dark .x-field-select .x-component-outer, .x-toolbar-dark .x-field-select .x-component-outer.x-button-back::after, .x-toolbar-dark .x-field-select .x-component-outer.x-button-forward::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer.x-button-back::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer.x-button-forward::after, .x-toolbar-dark .x-field-select .x-component-outer::before, .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-back::after, .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-forward::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-back::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-forward::after {
background-color: #303030;
background-image: none;
}
.x-toolbar-dark .x-button, .x-toolbar .x-toolbar-dark .x-button, .x-toolbar-dark .x-field-select .x-component-outer, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer, .x-toolbar-dark .x-field-select .x-component-outer::before, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before {
border: 1px solid #000000;
border-top-color: #000000;
color: white;
}
.x-toolbar-dark .x-button.x-button-pressing, .x-toolbar-dark .x-button.x-button-pressing::after, .x-toolbar-dark .x-button.x-button-pressed, .x-toolbar-dark .x-button.x-button-pressed::after, .x-toolbar-dark .x-button.x-button-active, .x-toolbar-dark .x-button.x-button-active::after, .x-toolbar .x-toolbar-dark .x-button.x-button-pressing, .x-toolbar .x-toolbar-dark .x-button.x-button-pressing::after, .x-toolbar .x-toolbar-dark .x-button.x-button-pressed, .x-toolbar .x-toolbar-dark .x-button.x-button-pressed::after, .x-toolbar .x-toolbar-dark .x-button.x-button-active, .x-toolbar .x-toolbar-dark .x-button.x-button-active::after, .x-toolbar-dark .x-field-select .x-component-outer.x-button-pressing, .x-toolbar-dark .x-field-select .x-component-outer.x-button-pressing::after, .x-toolbar-dark .x-field-select .x-component-outer.x-button-pressed, .x-toolbar-dark .x-field-select .x-component-outer.x-button-pressed::after, .x-toolbar-dark .x-field-select .x-component-outer.x-button-active, .x-toolbar-dark .x-field-select .x-component-outer.x-button-active::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer.x-button-pressing, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer.x-button-pressing::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer.x-button-pressed, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer.x-button-pressed::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer.x-button-active, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer.x-button-active::after, .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-pressing, .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-pressing::after, .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-pressed, .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-pressed::after, .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-active, .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-active::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-pressing, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-pressing::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-pressed, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-pressed::after, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-active, .x-toolbar .x-toolbar-dark .x-field-select .x-component-outer::before.x-button-active::after {
background-color: #383838;
background-image: none;
}
/*Done Button styles overriding*/
.x-button.x-button-action, .x-button.x-button-action.x-button-back::after, .x-button.x-button-action.x-button-forward::after, .x-toolbar .x-button.x-button-action, .x-toolbar .x-button.x-button-action.x-button-back::after, .x-toolbar .x-button.x-button-action.x-button-forward::after, .x-button.x-button-action-round, .x-button.x-button-action-round.x-button-back::after, .x-button.x-button-action-round.x-button-forward::after, .x-toolbar .x-button.x-button-action-round, .x-toolbar .x-button.x-button-action-round.x-button-back::after, .x-toolbar .x-button.x-button-action-round.x-button-forward::after, .x-button.x-button-action-small, .x-button.x-button-action-small.x-button-back::after, .x-button.x-button-action-small.x-button-forward::after, .x-toolbar .x-button.x-button-action-small, .x-toolbar .x-button.x-button-action-small.x-button-back::after, .x-toolbar .x-button.x-button-action-small.x-button-forward::after {
background-color: #303030;
background-image: none;
}
.x-button.x-button-action, .x-toolbar .x-button.x-button-action, .x-button.x-button-action-round, .x-toolbar .x-button.x-button-action-round, .x-button.x-button-action-small, .x-toolbar .x-button.x-button-action-small {
border: 1px solid #000000;
border-top-color: #000000;
color: white;
}
.x-button.x-button-action.x-button-pressing, .x-button.x-button-action.x-button-pressing::after, .x-button.x-button-action.x-button-pressed, .x-button.x-button-action.x-button-pressed::after, .x-button.x-button-action.x-button-active, .x-button.x-button-action.x-button-active::after, .x-toolbar .x-button.x-button-action.x-button-pressing, .x-toolbar .x-button.x-button-action.x-button-pressing::after, .x-toolbar .x-button.x-button-action.x-button-pressed, .x-toolbar .x-button.x-button-action.x-button-pressed::after, .x-toolbar .x-button.x-button-action.x-button-active, .x-toolbar .x-button.x-button-action.x-button-active::after, .x-button.x-button-action-round.x-button-pressing, .x-button.x-button-action-round.x-button-pressing::after, .x-button.x-button-action-round.x-button-pressed, .x-button.x-button-action-round.x-button-pressed::after, .x-button.x-button-action-round.x-button-active, .x-button.x-button-action-round.x-button-active::after, .x-toolbar .x-button.x-button-action-round.x-button-pressing, .x-toolbar .x-button.x-button-action-round.x-button-pressing::after, .x-toolbar .x-button.x-button-action-round.x-button-pressed, .x-toolbar .x-button.x-button-action-round.x-button-pressed::after, .x-toolbar .x-button.x-button-action-round.x-button-active, .x-toolbar .x-button.x-button-action-round.x-button-active::after, .x-button.x-button-action-small.x-button-pressing, .x-button.x-button-action-small.x-button-pressing::after, .x-button.x-button-action-small.x-button-pressed, .x-button.x-button-action-small.x-button-pressed::after, .x-button.x-button-action-small.x-button-active, .x-button.x-button-action-small.x-button-active::after, .x-toolbar .x-button.x-button-action-small.x-button-pressing, .x-toolbar .x-button.x-button-action-small.x-button-pressing::after, .x-toolbar .x-button.x-button-action-small.x-button-pressed, .x-toolbar .x-button.x-button-action-small.x-button-pressed::after, .x-toolbar .x-button.x-button-action-small.x-button-active, .x-toolbar .x-button.x-button-action-small.x-button-active::after {
background-color: #383838;
background-image: none;
}