When you install Bootstrap 3 into the Content folder, it includes a folder structure like this:
~/Content/css/boostrap.min.css
~/Content/fonts/glyphincons-halflinkgs-regular.(eot|svg|ttf|woff)
The 403 "Forbidden" error, is likely because the default CSS bundle route "~/Content/css" matches a valid folder name, and MVC cannot resolve the conflict between a route and folder.
Fix the 403 by renaming your route to something that doesn't match a real folder like, "~/Content/cssbundle".
When you change the bundle name, the 404 "file not found" error occurs for the fonts as the relative path from the CSS files to the fonts is no longer valid.
Fix the 404 by moving your fonts folder to the root of your project. (I wish I had a better solution here)