diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index b11619088..8d390d6f7 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -863,7 +863,7 @@ module ApplicationHelper
   def body_css_classes
     css = []
     if theme = Redmine::Themes.theme(Setting.ui_theme)
-      css << 'theme-' + theme.name
+      css << 'theme-' + theme.name.tr(' ', '_')
     end
 
     css << 'project-' + @project.identifier if @project && @project.identifier.present?
diff --git a/test/integration/lib/redmine/themes_test.rb b/test/integration/lib/redmine/themes_test.rb
index f5fbc89b6..42cacd172 100644
--- a/test/integration/lib/redmine/themes_test.rb
+++ b/test/integration/lib/redmine/themes_test.rb
@@ -101,4 +101,12 @@ class ThemesTest < Redmine::IntegrationTest
   ensure
     Redmine::Utils.relative_url_root = ''
   end
+
+  def test_body_css_class_with_spaces_in_theme_name
+    @theme.instance_variable_set(:@name, 'Foo bar baz')
+    get '/'
+
+    assert_response :success
+    assert_select 'body[class~="theme-Foo_bar_baz"]'
+  end
 end