Thank you for contacting Telerik Support.
First of all, make sure you registered
your theme correctly in the Administration section of Sitefinity. Go to Administration
-> Frontend Themes
(~/App_Data/Sitefinity/WebsiteTemplates/testTheme/App_Themes/original/). The 403 exception may have occurred, because you tried to link directly to resources in the App_Data folder, which is not possible, since this folder has special permissions.
If that's not it, try renaming your stylesheet to main.css. Sitefinity should load it automatically.
You could also try to move your theme outside the App_Data file and see if you can load it.
Let me know whether the proposed solutions help.
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