add missing text/css content type

crates/treehouse/src/vfs/physical.rs

@@ -95,6 +95,7 @@ fn guess_content_type(extension: &str) -> Option<&'static str> {
     match extension {
         "html" => Some("text/html"),
         "js" => Some("text/javascript"),
+        "css" => Some("text/css"),
         "woff" => Some("font/woff2"),
         "svg" => Some("image/svg+xml"),
         "atom" => Some("application/atom+xml"),