update docs to include THEMES config value (#5752)

* update docs to include THEMES

* Update app.ini.sample

* Update config-cheat-sheet.en-us.md

custom/conf/app.ini.sample

 SHOW_USER_EMAIL = true
 ; Set the default theme for the Gitea install
 DEFAULT_THEME = gitea
-; All available themes
+; All available themes. Allow users select personalized themes regardless of the value of `DEFAULT_THEME`.
 THEMES = gitea,arc-green
 
 [ui.admin]

docs/content/doc/advanced/config-cheat-sheet.en-us.md

 - `FEED_MAX_COMMIT_NUM`: **5**: Number of maximum commits shown in one activity feed.
 - `GRAPH_MAX_COMMIT_NUM`: **100**: Number of maximum commits shown in the commit graph.
 - `DEFAULT_THEME`: **gitea**: \[gitea, arc-green\]: Set the default theme for the Gitea install.
+- `THEMES`:  **gitea,arc-green**: All available themes. Allow users select personalized themes
+  regardless of the value of `DEFAULT_THEME`.
 
 ### UI - Admin (`ui.admin`)
 

docs/content/doc/usage/https-support.md

 * [apache2/httpd](https://httpd.apache.org/docs/2.4/ssl/ssl_howto.html)
 * [caddy](https://caddyserver.com/docs/tls)
 
-Note: You connection between your reverse proxy and gitea might be unencrypted. To encrypt it too follow the [built-in server guide](#using-built-in-server) and change
+Note: Your connection between your reverse proxy and gitea might be unencrypted. To encrypt it too follow the [built-in server guide](#using-built-in-server) and change
 the proxy url to `https://[URL]`.