That's actually pretty simple, and is what CSS is meant to be helping with - there will be a pain of having to wrangle the framework's HTML to fit what you need, but it's doable, and a small price to pay for the benefits the tried-and-tested framework brings.
If you want to use for example Tailwind CSS classes in your blog you also need to integrate it with your build pipeline to add/remove unused classes from the final stylesheet.
I thought Sitepress looks interesting, as its supposed to integrate with Rails. Have you given that one a try?
I expected there to be many solutions for this problem but wasn't able to find a simple one.
In the end I just generated a simple "post" model and have some ugly admin interface that allows me to paste some markdown into a small box and publish it. Then I'm just using some gems for RSS feed / sitemap handling.
Basically, just convert your markdown to HTML, and dump that into your existing templates.
If you are trying to have your blog as your main website and not just one sub-category it makes sense to use a blogging framework (Hugo, Ghost, Wordpress etc.), but I realized that having a blog integrated in a product isn't so straight forward if you want to keep your existing footer, header, CSS and share it.
I tried to use Hugo to integrate a blog in my Rails app but it was a bigger amount of pain than just doing it quickly in your existing code base with all the tools and styles already available.