Excuse me for the misunderstanding. No, you can not add additional controls as you want to do. The best way is to try to create a new skin for the controls you like to m customize and to apply the same color schemes you have applied already to the grid, slider and calendar.
The new controls skinning will have the same name as the name used for the other controls and once you have done, you will combine them into a common folder. If your skin is named - mySkin - now you have a folder called mySkin and inside it there are CSS files door grid, slider and calendar, and filder containing the necessary images for these controls. You could add in mySkin folder controls that were created later, but should keep the same skin name convention.
the Telerik team
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