NGINX sub_filter problem

I'm currently setting up a reverse proxy using nginx which for the most part is working perfectly. The conf file has the following entry: server { listen [login to view URL] default_server; location /[login to view URL] { proxy_set_header Accept-Encoding ""; proxy_set_header Host [login to view URL]; proxy_pass https://1.2.3.4/; sub_filter_types text/html text/css text/javascript; sub_filter "[login to view URL]" "[login to view URL]"; sub_filter_once off; } } The proxy is successfully sending back the page and serving elements from the correct proxy locations, however this is where the problem begins. The style-sheet ([login to view URL]) loads perfectly through the proxy, but inside that, there is a style import line as follows: @import "[login to view URL]"; As you can see, the domain is not being filterd/changed by the sub_filter at all inside the CSS file. I have tried changing sub_filter types to the following with no luck: sub_filter_types *; Should the filter be changing the domain in the CSS file? We need you to use your NGINX expertise to make sure the contents of the CSS file are also being modified by the filter.