Hello Vahid,

Any change or customization to the style or template of RadComboBox will hard-code resources, regardless of whether it is for Office Black or for another theme. This is inevitable.

If you wish to use more than one theme in your application and to keep changing themes at runtime, the approach you should use is not elegant and will produce a large amount of resources. 

First off, you have to apply your customizations to the RadComboBox style for each Telerik theme. Then, you can either: switch resource dictionaries at runtime, depending on the chosen theme, or you can: create a custom theme corresponding to each Telerik theme and apply it to the RadComboBox control depending on the switched application theme.

As explained, this is not recommended, so it is better to  have a custom RadComboBox with a single theme if this fits in your scenario.

In any case, you will need to also recreate the layout in order for your controls to pick up the new theme.

I hope this helps.

Regards,
Dani
the Telerik team