There is no way to apply a single paragraph attribute.

To demonstrate the issue, use the latest Monkey Tester
https://github.com/andy-goryachev-oracle/MonkeyTest

- select the RichTextArea page
- select the RichTextArea model
- select Control -> Context Menu -> RichTextArea menu
- type in several paragraphs
- select all and apply one paragraph attribute, let's say Paragraph Styles -> Background -> (any non-null)
- select a subset of previously selected paragraphs and apply a second paragraph attribute, let's say Paragraph Styles -> Bullet -> (any non-null)

Observe that the previously set paragraph styles are removed. We are missing
  protected abstract StyledTextModel.applyParagraphStyle(int index, StyleAttributeMap paragraphAttrs);
to complement the existing
  protected abstract void setParagraphStyle(int index, StyleAttributeMap paragraphAttrs);

See StyledTextModel L799