FIX: emails should use default theme colors

app/models/color_scheme.rb

@@ -112,15 +112,8 @@ class ColorScheme < ActiveRecord::Base
   end
 
   def self.lookup_hex_for_name(name)
-    Discourse.plugin_themes.each do |pt|
-      if pt.color_scheme
-        found = pt.color_scheme[name.to_sym]
-        return found if found
-      end
-    end
-
-    # Can't use `where` here because base doesn't allow it
-    (base).colors.find {|c| c.name == name }.try(:hex) || :nil
+    enabled_color_scheme = Theme.where(key: SiteSetting.default_theme_key).first&.color_scheme
+    (enabled_color_scheme || base).colors.find {|c| c.name == name }.try(:hex) || :nil
   end
 
   def self.hex_for_name(name)

spec/mailers/user_notifications_spec.rb

@@ -244,6 +244,26 @@ describe UserNotifications do
         expect(html).to_not include hidden.raw
         expect(html).to_not include user_deleted.raw
       end
+
+      it "uses theme color" do
+        cs = Fabricate(:color_scheme, name: 'Fancy', color_scheme_colors: [
+          Fabricate(:color_scheme_color, name: 'header_primary',  hex: 'F0F0F0'),
+          Fabricate(:color_scheme_color, name: 'header_background', hex: '1E1E1E'),
+          Fabricate(:color_scheme_color, name: 'tertiary', hex: '858585')
+        ])
+        theme = Theme.create!(
+          name: 'my name',
+          user_id: Fabricate(:admin).id,
+          user_selectable: true,
+          color_scheme_id: cs.id
+        )
+        theme.set_default!
+
+        html = subject.html_part.body.to_s
+        expect(html).to include 'F0F0F0'
+        expect(html).to include '1E1E1E'
+        expect(html).to include '858585'
+      end
     end
 
   end