Fix the headings hierarchy in the Styles sidebar

[afercia]

Description

Follow-up to #42373

The headings hierarchy in the Styles sidebar is less than ideal. It skips levels, doesn't follow a logical structure and it appears it was implemented more for visual purposes rather than for good semantics.

Suffice to say the first heading within the Styles sidebar is a H5, which doesn't make sense and it's totally disconnected from the overall headings hierarchy.

A good headings structure is not just about good semantics. Headings are also a navigational tool for screen reader users, as they can use dedicated commands to jump through headings, get a sense of the overall content, and navigate a page. This principle applies also to administration user interfaces, where headings can be used to identify the main parts of the interface.

Step-by-step reproduction instructions

• Go to the Site editor.
• Open the Styles sidebar.
• Click 'Colors'.
• Check the 'Colors' text to the right of the back button is a H5 heading. This is incorrect.
• Check that 'Palette' and 'Elements' are H2 headings. This doesn't communicate a proper hierarchy.
• Click 'Buttons'.
• Check the 'Buttons' text to the right of the back button is a H5 heading. This is incorrect.
• Check the 'Text color' heading underneath 'Buttons' is a H4 heading. This is incorrect because 'Text color' should be a sub-heading of 'Buttons'.
• Check the all-caps text 'THEME' is a H2 heading. It should be a sub-heading of 'Text color'.
• Check the all-caps text 'DEFAULT' is a H2 heading. It should be a sub-heading of 'Text color'.
• Check 'Background color' is a H2 heading. It skips a heading level but it should probably be a H2 in the first place.
• Check the all-caps text 'THEME' is a H2 heading. It should be a sub-heading of 'Background color'.
• Check the all-caps text 'DEFAULT' is a H2 heading. It should be a sub-heading of 'Background color'.

Hint; Use the HeadingsMap browser extension to check the headings hierarchy.

Screenshots, screen recording, code snippet

Worth mentioning the HeadingsMap browser extension only highlights skipped heading levels, and in the screenshot below there are a few headings highlighted as errors. It doesn't evaluate whether a heading level is logically correct or not, which requires evaluation by a human.

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes