recommends for newsletter-looking themes?

https://themes.wordpress.net – there are a ton of themes there you can choose from.

since all newsletters look different, it’s hard to tell what YOU mean specifically, so try searching there to find what you’re looking for.

Letterhead is a clean black/white design:
https://themes.wordpress.net/?view_type=filmstrip&item_type=search&page_number=3&count_per_page=12&item_filter=letterhead&previous_view_type=slideshow&previous_count_per_page=12
Couple of font changes, custom image in the header… ?

The https://webdesignbook.net/themes/ has a large list of themes to check out:

The Magazine: https://webdesignbook.net/themes/?p=407
Correspondence: https://webdesignbook.net/themes/?p=399
Plaintext: https://webdesignbook.net/themes/?p=346
or
Cub Reporter: https://webdesignbook.net/themes/?p=199

Good Luck!

Thanks for the recommends. They look pretty plain, though. I was hoping to find a design that looks like https://www.nytimes.com, which I think recently changed into a blog.

Calling NYTimes a “newsletter” is a bit of understatement, I guess. Seemingly what you are looking for is a newspaper look in a theme. There are over 700 themes that I know of – so I imagine there might be something like that.