diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 7c36bc6b8..08cca0734 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -10,6 +10,7 @@ class ApplicationController < ActionController::Base
   include SessionTrackingConcern
   include CacheConcern
   include DomainControlHelper
+  include ThemingConcern
 
   helper_method :current_account
   helper_method :current_session
@@ -73,75 +74,6 @@ class ApplicationController < ActionController::Base
     new_user_session_path
   end
 
-  def pack(data, pack_name, skin = 'default')
-    return nil unless pack?(data, pack_name)
-    pack_data = {
-      common: pack_name == 'common' ? nil : resolve_pack(data['name'] ? Themes.instance.flavour(current_flavour) : Themes.instance.core, 'common', skin),
-      flavour: data['name'],
-      pack: pack_name,
-      preload: nil,
-      skin: nil,
-      supported_locales: data['locales'],
-    }
-    if data['pack'][pack_name].is_a?(Hash)
-      pack_data[:common] = nil if data['pack'][pack_name]['use_common'] == false
-      pack_data[:pack] = nil unless data['pack'][pack_name]['filename']
-      if data['pack'][pack_name]['preload']
-        pack_data[:preload] = [data['pack'][pack_name]['preload']] if data['pack'][pack_name]['preload'].is_a?(String)
-        pack_data[:preload] = data['pack'][pack_name]['preload'] if data['pack'][pack_name]['preload'].is_a?(Array)
-      end
-      if skin != 'default' && data['skin'][skin]
-        pack_data[:skin] = skin if data['skin'][skin].include?(pack_name)
-      else  #  default skin
-        pack_data[:skin] = 'default' if data['pack'][pack_name]['stylesheet']
-      end
-    end
-    pack_data
-  end
-
-  def pack?(data, pack_name)
-    if data['pack'].is_a?(Hash) && data['pack'].key?(pack_name)
-      return true if data['pack'][pack_name].is_a?(String) || data['pack'][pack_name].is_a?(Hash)
-    end
-    false
-  end
-
-  def nil_pack(data, pack_name, skin = 'default')
-    {
-      common: pack_name == 'common' ? nil : resolve_pack(data['name'] ? Themes.instance.flavour(current_flavour) : Themes.instance.core, 'common', skin),
-      flavour: data['name'],
-      pack: nil,
-      preload: nil,
-      skin: nil,
-      supported_locales: data['locales'],
-    }
-  end
-
-  def resolve_pack(data, pack_name, skin = 'default')
-    result = pack(data, pack_name, skin)
-    unless result
-      if data['name'] && data.key?('fallback')
-        if data['fallback'].nil?
-          return nil_pack(data, pack_name, skin)
-        elsif data['fallback'].is_a?(String) && Themes.instance.flavour(data['fallback'])
-          return resolve_pack(Themes.instance.flavour(data['fallback']), pack_name)
-        elsif data['fallback'].is_a?(Array)
-          data['fallback'].each do |fallback|
-            return resolve_pack(Themes.instance.flavour(fallback), pack_name) if Themes.instance.flavour(fallback)
-          end
-        end
-        return nil_pack(data, pack_name, skin)
-      end
-      return data.key?('name') && data['name'] != Setting.default_settings['flavour'] ? resolve_pack(Themes.instance.flavour(Setting.default_settings['flavour']), pack_name) : nil_pack(data, pack_name, skin)
-    end
-    result
-  end
-
-  def use_pack(pack_name)
-    @core = resolve_pack(Themes.instance.core, pack_name)
-    @theme = resolve_pack(Themes.instance.flavour(current_flavour), pack_name, current_skin)
-  end
-
   protected
 
   def truthy_param?(key)
diff --git a/app/controllers/concerns/theming_concern.rb b/app/controllers/concerns/theming_concern.rb
new file mode 100644
index 000000000..1ee3256c0
--- /dev/null
+++ b/app/controllers/concerns/theming_concern.rb
@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+module ThemingConcern
+  extend ActiveSupport::Concern
+
+  def use_pack(pack_name)
+    @core = resolve_pack_with_common(Themes.instance.core, pack_name)
+    @theme = resolve_pack_with_common(Themes.instance.flavour(current_flavour), pack_name, current_skin)
+  end
+
+  private
+
+  def valid_pack_data?(data, pack_name)
+    data['pack'].is_a?(Hash) && [String, Hash].any? { |c| data['pack'][pack_name].is_a?(c) }
+  end
+
+  def nil_pack(data)
+    {
+      use_common: true,
+      flavour: data['name'],
+      pack: nil,
+      preload: nil,
+      skin: nil,
+      supported_locales: data['locales'],
+    }
+  end
+
+  def pack(data, pack_name, skin)
+    pack_data = {
+      use_common: true,
+      flavour: data['name'],
+      pack: pack_name,
+      preload: nil,
+      skin: nil,
+      supported_locales: data['locales'],
+    }
+
+    return pack_data unless data['pack'][pack_name].is_a?(Hash)
+
+    pack_data[:use_common] = false if data['pack'][pack_name]['use_common'] == false
+    pack_data[:pack] = nil unless data['pack'][pack_name]['filename']
+
+    preloads = data['pack'][pack_name]['preload']
+    pack_data[:preload] = [preloads] if preloads.is_a?(String)
+    pack_data[:preload] = preloads if preloads.is_a?(Array)
+
+    if skin != 'default' && data['skin'][skin]
+      pack_data[:skin] = skin if data['skin'][skin].include?(pack_name)
+    elsif data['pack'][pack_name]['stylesheet']
+      pack_data[:skin] = 'default'
+    end
+
+    pack_data
+  end
+
+  def resolve_pack(data, pack_name, skin)
+    return pack(data, pack_name, skin) if valid_pack_data?(data, pack_name)
+    return if data['name'].blank?
+
+    fallbacks = []
+    if data.key?('fallback')
+      fallbacks = data['fallback'] if data['fallback'].is_a?(Array)
+      fallbacks = [data['fallback']] if data['fallback'].is_a?(String)
+    elsif data['name'] != Setting.default_settings['flavour']
+      fallbacks = [Setting.default_settings['flavour']]
+    end
+
+    fallbacks.each do |fallback|
+      return resolve_pack(Themes.instance.flavour(fallback), pack_name) if Themes.instance.flavour(fallback)
+    end
+
+    nil
+  end
+
+  def resolve_pack_with_common(data, pack_name, skin = 'default')
+    result = resolve_pack(data, pack_name, skin) || nil_pack(data)
+    result[:common] = resolve_pack(data, 'common', skin) if result.delete(:use_common)
+    result
+  end
+end
diff --git a/app/lib/themes.rb b/app/lib/themes.rb
index 2147904e4..81e016d4a 100644
--- a/app/lib/themes.rb
+++ b/app/lib/themes.rb
@@ -14,16 +14,20 @@ class Themes
     result = Hash.new
     Dir.glob(Rails.root.join('app', 'javascript', 'flavours', '*', 'theme.yml')) do |path|
       data = YAML.load_file(path)
+      next unless data['pack']
+
       dir = File.dirname(path)
       name = File.basename(dir)
       locales = []
       screenshots = []
+
       if data['locales']
         Dir.glob(File.join(dir, data['locales'], '*.{js,json}')) do |locale|
           localeName = File.basename(locale, File.extname(locale))
           locales.push(localeName) unless localeName.match(/defaultMessages|whitelist|index/)
         end
       end
+
       if data['screenshot']
         if data['screenshot'].is_a? Array
           screenshots = data['screenshot']
@@ -31,38 +35,37 @@ class Themes
           screenshots.push(data['screenshot'])
         end
       end
-      if data['pack']
-        data['name'] = name
-        data['locales'] = locales
-        data['screenshot'] = screenshots
-        data['skin'] = { 'default' => [] }
-        result[name] = data
-      end
+
+      data['name'] = name
+      data['locales'] = locales
+      data['screenshot'] = screenshots
+      data['skin'] = { 'default' => [] }
+      result[name] = data
     end
 
     Dir.glob(Rails.root.join('app', 'javascript', 'skins', '*', '*')) do |path|
       ext = File.extname(path)
       skin = File.basename(path)
       name = File.basename(File.dirname(path))
-      if result[name]
-        if File.directory?(path)
-          pack = []
-          Dir.glob(File.join(path, '*.{css,scss}')) do |sheet|
-            pack.push(File.basename(sheet, File.extname(sheet)))
-          end
-        elsif ext.match(/^\.s?css$/i)
-          skin = File.basename(path, ext)
-          pack = ['common']
-        end
-        if skin != 'default'
-          result[name]['skin'][skin] = pack
+      next unless result[name]
+
+      if File.directory?(path)
+        pack = []
+        Dir.glob(File.join(path, '*.{css,scss}')) do |sheet|
+          pack.push(File.basename(sheet, File.extname(sheet)))
         end
+      elsif ext.match(/^\.s?css$/i)
+        skin = File.basename(path, ext)
+        pack = ['common']
+      end
+
+      if skin != 'default'
+        result[name]['skin'][skin] = pack
       end
     end
 
     @core = core
     @conf = result
-
   end
 
   def core
diff --git a/app/views/layouts/_theme.html.haml b/app/views/layouts/_theme.html.haml
index 92de64b0d..5dba77621 100644
--- a/app/views/layouts/_theme.html.haml
+++ b/app/views/layouts/_theme.html.haml
@@ -2,12 +2,13 @@
   - if theme[:pack] != 'common' && theme[:common]
     = render partial: 'layouts/theme', object: theme[:common]
   - if theme[:pack]
-    = javascript_pack_tag theme[:flavour] ? "flavours/#{theme[:flavour]}/#{theme[:pack]}" : "core/#{theme[:pack]}", crossorigin: 'anonymous'
+    - pack_path = theme[:flavour] ? "flavours/#{theme[:flavour]}/#{theme[:pack]}" : "core/#{theme[:pack]}"
+    = javascript_pack_tag pack_path, crossorigin: 'anonymous'
     - if theme[:skin]
       - if !theme[:flavour] || theme[:skin] == 'default'
-        = stylesheet_pack_tag theme[:flavour] ? "flavours/#{theme[:flavour]}/#{theme[:pack]}" : "core/#{theme[:pack]}", media: 'all', crossorigin: 'anonymous'
+        = stylesheet_pack_tag pack_path, media: 'all', crossorigin: 'anonymous'
       - else
-        = stylesheet_pack_tag "skins/#{theme[:flavour]}/#{theme[:skin]}/#{theme[:pack]}", crossorigin: 'anonymous'
+        = stylesheet_pack_tag "skins/#{theme[:flavour]}/#{theme[:skin]}/#{theme[:pack]}", media: 'all', crossorigin: 'anonymous'
     - if theme[:preload]
       - theme[:preload].each do |link|
         %link{ href: asset_pack_path("#{link}.js"), crossorigin: 'anonymous', rel: 'preload', as: 'script' }/