DEV: Use `frozen_string_literal: true` on `models/theme.rb`.

DEV: Use frozen_string_literal: true on models/theme.rb.

diff --git a/app/models/theme.rb b/app/models/theme.rb
index f8c486a..a96ef39 100644
--- a/app/models/theme.rb
+++ b/app/models/theme.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require_dependency 'distributed_cache'
 require_dependency 'stylesheet/compiler'
 require_dependency 'stylesheet/manager'

GitHub sha: 587cfd45