It would be a nice feature for the WindowService and DialogService settings to have a themeColor property.
Indeed, such a feature would be a good addition to the Kendo UI for Angular Dialogs.
A possible workaround is to use the cssClass property and add one that matches the desired theme.
The available classes for the Window are k-window-primary, k-window-dark, and k-window-light.
The available classes for the Dialog are k-dialog-primary, k-dialog-dark, and k-dialog-light.
I trust that the given workaround will be of assistance until the Dialogs package incorporates such a feature.