When a Rich Text Element is CMS-Bound, it has a purple border, and it can not be edited or styled in the Designer. This is because the sub-elements such as paragraphs and headings cannot be selected while it is CMS-bound.
You can un-bind it temporarily, to make those selections available, and then re-bind it.
Or, you can use a style-guide approach;
Make sure you Rich Text Element has a unique class
Create a draft static page in your site called Style Guide
On that page, create a Rich Text Element, and assign it the same class
Because this RTE is not CMS-bound, you can style the sub-elements as needed. Since it shares the same class, the changes you apply here will apply to your CMS-bound RTE as well.