<link rel="stylesheet" type="text/css" href="~/content.css" /> but still it doesnt work
Sounds like it is relative path issue
. At times, after deployment the path referenced as a source for images, stylesheets, etc is not correctly formed. This leads to no file found at the location referred because of which images don't download or style-sheets does not get applied.
You would need to make sure that the path is correct such that file is found and used.
I would suggest you to use this Tip and resolve the path correctly before setting the source of the file: Resolving Paths in a Multi-Folder WebSite