Hi Daniel Lindberg,
Thank you for contacting us. You are in the right direction to you use themes. If you want to apply any style to the calendar you could use an existing style and change it to your preference, i.e. change the style for the special days state. Load the attached files in the Visual Style Builder and find the differences from your theme. I hope this will help you.
If you have additional questions feel free to ask.
the Telerik team
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