#a11y-headings - Themes page heading fixes

[Cheffheid]

Each theme's name (in both the list and the theme details dialog) is an H3 that needs to be changed to an H2.

The theme details also list the theme's author with an H4, which needs to be bumped up to an H3.

Additionally, the "Update Available" text is also an H4. With the theme's author being changed to H3 already it can probably remain that way if there's any value to be had in keeping it a heading.

Like with the other heading tickets, ensure that the design doesn't change and ensure everything keeps working like it was before.

#a11y-headings

[joehills]

Changes the heading tags for theme name and author, 2 minor css changes to preserve styles

[joehills]

Same code changes, but generated patch form WordPress root instead of trunk. Apologies, first time committer.

[Cheffheid]

@joehills Looking good, thank you!

There's one more heading that would need to get changed (and I missed it on my initial description) and it's for the "Add New Theme" link block. Once that's done it should be good to go as far as I'm concerned.

[SergeyBiryukov]

In 33820:

Bump H3 headings to H2 on Themes screen for better accessibility.

props joehills.
fixes #33560.