Rename modern to default theme

BTCPayServer/HostedServices/CssThemeManager.cs

@@ -22,7 +22,7 @@ namespace BTCPayServer.HostedServices
         public void Update(ThemeSettings data)
         {
             if (String.IsNullOrWhiteSpace(data.ThemeCssUri))
-                _themeUri = "/main/themes/classic.css";
+                _themeUri = "/main/themes/default.css";
             else
                 _themeUri = data.ThemeCssUri;
 

BTCPayServer/Views/Server/Theme.cshtml

@@ -18,8 +18,8 @@
             <div class="form-group">
                 <label asp-for="ThemeCssUri"></label>
                 <select asp-for="ThemeCssUri" class="form-control">
+                    <option value="/main/themes/default.css">Default</option>
                     <option value="/main/themes/classic.css">Classic</option>
-                    <option value="/main/themes/modern.css">Modern</option>
                     <option value="/main/themes/casa.css">Casa</option>
                 </select>
                 <span asp-validation-for="ThemeCssUri" class="text-danger"></span>