It's not clear if what you want is to target certain elements with your css, or only load a specific css file on some pages (the newsletter ones).
Both are possible, but first let's make something clear.
With CSS you can apply the same style(s) from a single css file, to many elements, on as many as you want pages. The more important condition is to target your elements properly and for this it's recommended to be consistent with how you use the class attributes in your markup.
The best practice is to use the class attributes instead of the id to style your html.
I don't know how your newsletter articles are structured/built, nor I can expand too much on delivering an HTML/CSS tutorial here, but if for example each of your newsletter page/article has a wrapper div element with a unique ID, and you try to bind your styles on that ID, then this isn't going to work.
Well.. it's late here and I feel tired same time that I started thinking all the possible different options depending on different use-cases - I think your question is too broad. So maybe either send a link to your website/newsletter pages, or add more details to narrow down the use-cases, so to get a more specific answer.