Thank you for contacting Telerik Support.
Having only the information you provided, makes it rather hard to determine the root of the problem. Using application relative paths, when registering themes is very important and it seems that you used them correctly. Now please check the reading permissions of the folder in which you have placed your theme. Such issue may occur if the folder doesn't have read permissions. I would also advise you to make sure that you have configured your IIS according to this article: Configuring the IIS to host Sitefinity projects
. Take a look at this link
Let me know if the above information helps.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items