It’s pretty easy to do, yay! Give the RTE a class (like blogRTE 
), select the H2, then select ‘all H2 headings’.
You’ll see a box appear that says ‘nested selector in RTE’. When you click that, your changes only apply to your H2s in that rich text box 
. Make sure to remove any binding you had with a collection item before doing this. If you don’t unlink the RTE from the collection this option doesn’t work.
Here’s a link to the webflow university about RTEs Rich text | Webflow University