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

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

app/helpers/portal_helper.rb

@@ -0,0 +1,11 @@
+module PortalHelper
+  def generate_portal_bg_color(portal_color, theme)
+    base_color = theme == 'dark' ? 'black' : 'white'
+    "color-mix(in srgb, #{portal_color} 10%, #{base_color})"
+  end
+
+  def generate_portal_bg(portal_color, theme)
+    bg_image = theme == 'dark' ? 'grid_dark.svg' : 'grid.svg'
+    "background: url(/assets/images/hc/#{bg_image}) #{generate_portal_bg_color(portal_color, theme)}"
+  end
+end