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

custom/conf/app.ini.sample

@@ -75,6 +75,8 @@ THEME_COLOR_META_TAG = `#6cc644`
 MAX_DISPLAY_FILE_SIZE = 8388608
 ; Whether the email of the user should be shown in the Explore Users page
 SHOW_USER_EMAIL = true
+; Set the default theme for the Gitea install
+DEFAULT_THEME = gitea
 
 [ui.admin]
 ; Number of users that are displayed on one page