Add the ability to have built in themes in Gitea (#4198)

This makes it easier for user who want to theme but
don't have the ability to know how to customize templates
all that is required is a change in a config option

The reason why I chose the DEFAULT_THEME as variable,
as perhaps in the future we will allow users to chose their
theme whon logged in just like we do with languages

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

@@ -68,6 +68,7 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `EXPLORE_PAGING_NUM`: **20**: Number of repositories that are shown in one explore page.
 - `ISSUE_PAGING_NUM`: **10**: Number of issues that are shown in one page (for all pages that list issues).
 - `FEED_MAX_COMMIT_NUM`: **5**: Number of maximum commits shown in one activity feed.
+- `DEFAULT_THEME`: **gitea**: \[gitea, arc-green\]: Set the default theme for the Gitea install.
 
 ### UI - Admin (`ui.admin`)
 

docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -91,3 +91,7 @@ Apart from `extra_links.tmpl` and `extra_tabs.tmpl`, there are other useful temp
 ## Customizing gitignores, labels, licenses, locales, and readmes.
 
 Place custom files in corresponding sub-folder under `custom/options`.
+
+## Customizing the look of Gitea
+
+Gitea has two built-in themes, the default theme `gitea`, and a dark theme `arc-green`. To change the look of your Gitea install change the value of `DEFAULT_THEME` in the [ui](https://docs.gitea.io/en-us/config-cheat-sheet/#ui-ui) section of `app.ini` to another one of the available options.