feat: Adds dark theme support for public portal [CW-2525] (#7979)

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

app/controllers/public/api/v1/portals/base_controller.rb

@@ -1,5 +1,6 @@
 class Public::Api::V1::Portals::BaseController < PublicController
   before_action :show_plain_layout
+  before_action :set_color_scheme
   around_action :set_locale
   after_action :allow_iframe_requests
 
@@ -9,6 +10,14 @@ class Public::Api::V1::Portals::BaseController < PublicController
     @is_plain_layout_enabled = params[:show_plain_layout] == 'true'
   end
 
+  def set_color_scheme
+    @theme = if %w[dark light].include?(params[:theme])
+               params[:theme]
+             else
+               ''
+             end
+  end
+
   def set_locale(&)
     switch_locale_with_portal(&) if params[:locale].present?
     switch_locale_with_article(&) if params[:article_slug].present?