Safari extension for adding arbitrary Javascript to a page?

Is there a way to do this? I know I can use Stylish for adding arbitrary CSS to pages on a per-domain basis. Looking for something similar to add Javascript. For example, let's say I wanted to turn the text red for any h1 element on a particular site if the content of that h1 element is "News". Can't do that with CSS, need to do it with Javascript. Is there any extension I can use to add this? Or would I have to write my own?