The tricky part is that RadCheckBox inherits most of its behavior and properties from the RadToggleButton control. Thus the property you should use to check if the checkbox is checked is called Telerik.WinControls.UI.RadToggleButtonElement.ToggleState
In order to create a theme based on this property you should do the following:
- Load the RadCheckBox control in the VSB (you can optionally load the default theme as well)
- Select the Fill primitive inside the Checkmark element. Hint - zooming the preview would get you there much easier.
- Add a new state and click the "edit" icon button on the state tab-item to open the state editor dialog.
- In the "apply condition" section select the split button's "Show Condition editor" option - the Condition builder dialog should appear
- In the property box, select or type in the full prop name , i.e.:
Telerik.WinControls.UI.RadToggleButtonElement.ToggleState and select "On" for the Value
Now the customizations you do to the properties of the fill-primitive would apply only when the "ToggleState = On" condition is met.
Attached is an example theme that you may use as a starter.