diff --git a/lib/puppet/functions/validate_available_themes.rb b/lib/puppet/functions/validate_available_themes.rb
new file mode 100644
index 00000000..346050bd
--- /dev/null
+++ b/lib/puppet/functions/validate_available_themes.rb
@@ -0,0 +1,15 @@
+Puppet::Functions.create_function(:validate_available_themes) do
+
+  def validate_available_themes(themes)
+    req_keys = Set.new(['name', 'label', 'path'])
+    themes.each do |theme|
+      if theme.keys.to_set != req_keys
+        if theme.keys.to_set.subset?(req_keys)
+          raise Puppet::Error, "Some of the required keys (name, label and path) are missing"
+        else
+          raise Puppet::Error, "Unsupported keys are detected"
+        end
+      end
+    end
+  end
+end
diff --git a/manifests/init.pp b/manifests/init.pp
index ef11e5c9..ff6ff8b6 100644
--- a/manifests/init.pp
+++ b/manifests/init.pp
@@ -352,7 +352,7 @@
 #  [*available_themes*]
 #    (optional) An array of hashes detailing available themes. Each hash must
 #    have the followings keys for themes to be made available; name, label,
-#    path. Defaults to false
+#    path. Defaults to undef
 #
 #    { 'name' => 'theme_name', 'label' => 'theme_label', 'path' => 'theme_path' }
 #
@@ -364,14 +364,15 @@
 #      ]
 #    }
 #
-#   Or in Hiera:
-#   horizon::available_themes:
-#     - { name: 'default', label: 'Default', path: 'themes/default' }
-#     - { name: 'material', label: 'Material', path: 'themes/material' }
+#    Or in Hiera:
+#     horizon::available_themes:
+#      - { name: 'default', label: 'Default', path: 'themes/default' }
+#      - { name: 'material', label: 'Material', path: 'themes/material' }
 #
 #  [*default_theme*]
-#   (optional) The default theme to use from list of available themes. Value should be theme_name.
-#   Defaults to false
+#    (optional) The default theme to use from list of available themes. Value
+#    should be theme_name.
+#    Defaults to undef
 #
 #  [*authentication_plugins*]
 #    (optional) List of authentication plugins to be used.
@@ -610,8 +611,8 @@ class horizon(
   Boolean $secure_cookies                           = false,
   $django_session_engine                            = undef,
   $vhost_extra_params                               = undef,
-  $available_themes                                 = false,
-  $default_theme                                    = false,
+  Horizon::AvailableThemes $available_themes        = undef,
+  Optional[String[1]] $default_theme                = undef,
   Array[String[1]] $authentication_plugins          = [],
   Enum['on', 'off'] $password_autocomplete          = 'off',
   $create_image_defaults                            = undef,
@@ -657,6 +658,10 @@ class horizon(
     fail('websso_initial_choice is required when websso_choices_hide_keystone is true')
   }
 
+  if $available_themes {
+    validate_available_themes($available_themes)
+  }
+
   Service <| title == 'memcached' |> -> Class['horizon']
 
   if $policy_files_path != undef {
diff --git a/spec/type_aliases/availablethemes_spec.rb b/spec/type_aliases/availablethemes_spec.rb
new file mode 100644
index 00000000..d5874d72
--- /dev/null
+++ b/spec/type_aliases/availablethemes_spec.rb
@@ -0,0 +1,38 @@
+require 'spec_helper'
+
+describe 'Horizon::AvailableThemes' do
+  describe 'valid types' do
+    context 'with valid types' do
+      [
+        [
+          {'name' => 'default'}
+        ],
+        [
+          {'name' => 'default'},
+          {'name' => 'custom'}
+        ],
+      ].each do |value|
+        describe value.inspect do
+          it { is_expected.to allow_value(value) }
+        end
+      end
+    end
+  end
+
+  describe 'invalid types' do
+    context 'with garbage inputs' do
+      [
+        [],
+        ['name'],
+        [{'name' => 1}],
+        [{1 => 'default'}],
+        [{'name' => ''}],
+        [{'' => 'default'}],
+      ].each do |value|
+        describe value.inspect do
+          it { is_expected.not_to allow_value(value) }
+        end
+      end
+    end
+  end
+end
diff --git a/types/availablethemes.pp b/types/availablethemes.pp
new file mode 100644
index 00000000..583721f1
--- /dev/null
+++ b/types/availablethemes.pp
@@ -0,0 +1,6 @@
+type Horizon::AvailableThemes = Optional[
+  Array[
+    Hash[String[1], String[1]],
+    1
+  ]
+]