Changing header sizes on blog page, but keeping them as h2, etc. for SEO

Hello!

I'm working on a client's blog (currently not published) and she really doesn't like the huge size of her H2-H4 for her blog posts.

While I'm able to use assign custom styles to decrease their size without doing it globally, I checked the code and they lose their "H2, H3, H4" status. Because we are doing the blog partially for SEO purposes, I feel we'll lose out on a lot of opportunities if we're losing these.

Is there any code I could do instead? I tried doing various class and ID selectors so I could change just the size for blog posts, while keeping the H's, but not a single one worked. I'm not sure why, I couldn't seem to select anything. Is there some sort of trick for selecting things with HTML or CSS in the blogs? 

A few approaches I tried:

Selecting the h2 titles of certain sections

section[data-section-id="63cff1814c84ba211416e2ff"] h2 {
  font-size: 1.5rem ;
}

Selecting the h's of specific divs

#yui_3_17_2_1_1718391594494_5333 h2,
#yui_3_17_2_1_1718391594494_5333 h3,
#yui_3_17_2_1_1718391594494_5333 h4 {
  font-size: 1.5rem !important;
}

I tried putting each of these separately in both 

Page Header Code Injection

and then

Post Blog Item Code Injection

And no luck.

I've never had a problem using different methods to select things for CSS on "regular" Squarespace pages, so I wonder if I'm missing something? Thank you!

To target h2 on blog posts, you can use this

[class*="type-blog"] h2 {
	font-size: 20px !important;
}

Thank you so much! I didn't have any luck. Here's an example of an H3.
 

I tried to change it to 5px just to see if it would get teeny tiny and I could know if I was selecting it, but the size didn't change. I tried putting the code (modified from h2 to h3) in both the page header code injection and post blog item code injection, see below

example:

any ideas?

With Post Blog Item Code Injection, you need to use style tag

<style>
  [class*="type-blog"] h3 {
	font-size: 20px !important;
}
</style>