Hi Monica,
The theme could have changed some of it's underlying structure.
You are, of course, welcome to use the header options if it helps you change the size of the font - because you're right: screen readers are an edge case. In our situation (WordPress.com, I mean), we really have to stick by those best practices; our users often don't. I always encourage best practices, but this is one of those cases where you're PROBABLY ok. One thing to watch out for would be if you switched themes and headers were handled completely differently.
So if you'd like to give in-line styles a go, you can do this:
*Go to edit a post like you normally would, and switch to "text" view (instead of "visual")
*I've saved you a draft on your blog showing you in the text mode what to do: https://monicaglickman.wordpress.com/wp-admin/post.php?post=1521&action=edit&message=10
*You want to include <p style="font-size: XXem;">Your text</p>
*You can omit this for any "regular" text. The XX in the above can be anything you like. Ems are sensitive, so if it's set to 1.5em, 1.8 em is quite a bit bigger (and it's what I used in my example), but you'll probably want to change the decimal instead of the whole number :)
I hope that helps!