Expose default theme in meta and API (#13809)

Signed-off-by: jolheiser <john.olheiser@gmail.com>

Co-authored-by: 6543 <6543@obermui.de>

templates/base/head.tmpl

@@ -7,6 +7,7 @@
 	<title>{{if .Title}}{{.Title | RenderEmojiPlain}} - {{end}} {{if .Repository.Name}}{{.Repository.Name}} - {{end}}{{AppName}} </title>
 	<link rel="manifest" href="{{AppSubUrl}}/manifest.json" crossorigin="use-credentials">
 	<meta name="theme-color" content="{{ThemeColorMetaTag}}">
+	<meta name="default-theme" content="{{DefaultTheme}}" />
 	<meta name="author" content="{{if .Repository}}{{.Owner.Name}}{{else}}{{MetaAuthor}}{{end}}" />
 	<meta name="description" content="{{if .Repository}}{{.Repository.Name}}{{if .Repository.Description}} - {{.Repository.Description}}{{end}}{{else}}{{MetaDescription}}{{end}}" />
 	<meta name="keywords" content="{{MetaKeywords}}">

templates/swagger/v1_json.tmpl

@@ -13523,6 +13523,10 @@
             "type": "string"
           },
           "x-go-name": "AllowedReactions"
+        },
+        "default_theme": {
+          "type": "string",
+          "x-go-name": "DefaultTheme"
         }
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"