CSS formatting in a custom tag

Hi,

I searched around the forum and the web but I could not find an answer.

If I write some HTML with a <style> tag, I can get CSS formatting inside of it, working perfectly fine:

<style>
  .someClass{
    color:red;
  }
</style>

If I used a custom tag, it doesn't work (for instance with polymer):

<core-style>
  .someClass{
    color: red;
  }
</core-style>

I also have my own ploymer component for putting less css inline.

Is there a settings somewhere to say which HTML tag contains CSS / LESS / ... ?
Or maybe a plugin which can do that ?

Thanks,