Add CSS output formatter.

Jellyfin.Server/Extensions/ApiServiceCollectionExtensions.cs

@@ -77,6 +77,8 @@ namespace Jellyfin.Server.Extensions
                     opts.UseGeneralRoutePrefix(baseUrl);
                     opts.OutputFormatters.Insert(0, new CamelCaseJsonProfileFormatter());
                     opts.OutputFormatters.Insert(0, new PascalCaseJsonProfileFormatter());
+
+                    opts.OutputFormatters.Add(new CssOutputFormatter());
                 })
 
                 // Clear app parts to avoid other assemblies being picked up

Jellyfin.Server/Formatters/CssOutputFormatter.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc.Formatters;
+
+namespace Jellyfin.Server.Formatters
+{
+    /// <summary>
+    /// Css output formatter.
+    /// </summary>
+    public class CssOutputFormatter : TextOutputFormatter
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="CssOutputFormatter"/> class.
+        /// </summary>
+        public CssOutputFormatter()
+        {
+            SupportedMediaTypes.Clear();
+            SupportedMediaTypes.Add("text/css");
+
+            SupportedEncodings.Add(Encoding.UTF8);
+            SupportedEncodings.Add(Encoding.Unicode);
+        }
+
+        /// <summary>
+        /// Write context object to stream.
+        /// </summary>
+        /// <param name="context">Writer context.</param>
+        /// <param name="selectedEncoding">Unused. Writer encoding.</param>
+        /// <returns>Write stream task.</returns>
+        public override Task WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
+        {
+            return context.HttpContext.Response.WriteAsync(context.Object?.ToString());
+        }
+    }
+}