added nginx variable option

Setup.md

@@ -5,6 +5,7 @@
 <li><a href="#subfilter-method">Subfilter Method</a>
 <ul>
 <li><a href="#nginx">Nginx</a></li>
+<li><a href="#nginx-variable">Nginx Variable</a></li>
 <li><a href="#apache-untested">Apache</a></li>
 </ul>
 </li>
@@ -60,6 +61,70 @@ location /sonarr {
   }
 ```
 
+### Nginx Variable
+
+You can also setup Nginx to use variables to change the themes. This will update the theme on all your location blocks by just changing 1 variable. This is nice if you quickly want to change colors on all of your apps. 
+
+#### http block
+
+In you http block add the following:
+
+```nginx
+# THEME.PARK
+map $host $theme {
+    default <theme>;
+}
+```
+Change `<theme>` to the theme you want on all your apps. e.g. `default dark;`
+
+Available themes are:
+    `aquamarine`
+    `hotline`
+    `plex`
+    `dark`
+    `space-gray`
+
+#### theme-park.conf
+Next create a new file called `theme-park.conf` and add the following code: (Note: It's not really necessary to create the conf file but I think it looks cleaner in the location blocks)
+
+```nginx
+    proxy_set_header Accept-Encoding "";
+    sub_filter
+    '</head>'
+    '<link rel="stylesheet" type="text/css" href="https://gilbn.github.io/theme.park/CSS/themes/$app/$theme.css">
+    </head>';
+    sub_filter_once on;
+```
+As you can see the URL has variables in it `/$app/$theme.css`
+The `$theme` variable is set in the http block and will affect all server blocks. And the `$app` variable is set in the location block.
+    
+#### location blocks
+
+Now in the location block use the include syntax and include the theme-park.conf file and set the $app variable, like so:
+
+```nginx
+    set $app sonarr;
+    include /config/nginx/theme-park.conf;
+```
+
+**Example**
+```nginx
+location /sonarr {
+    proxy_pass http://192.168.1.34:8990/sonarr;
+    proxy_set_header X-Real-IP $remote_addr;
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    proxy_set_header X-Forwarded-Proto $scheme;
+    proxy_redirect off;
+    proxy_buffering off;
+    proxy_http_version 1.1;
+    proxy_no_cache $cookie_session;
+    set $app sonarr;
+    include /config/nginx/theme-park.conf;
+	}
+```
+
+Now when you change the variable in the http block and restart nginx, it will update the theme for all your apps!
+
 ### Apache (Untested)
 ```apache
 AddOutputFilterByType SUBSTITUTE text/html