Use new-style handbook theme for REST API Handbook

[rmccue]

I'm really not a fan of the ​REST API Handbook style. I really like the ​new (o2-based) Handbook style We should make the REST API Handbook look like the new Handbooks.

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This might be tough, since the newer style is based on the o2 style, but the older style is based on P2.

Filing this ticket to track anyway. If anyone has any ideas on how to make this happen, I'd love to know.

[jnylen0]

+1000, you can actually read the code samples in the CLI handbook, unlike in the above screenshot.

[coffee2code]

In 6512:

developer.wordpress.org: Adapt to new handbook styles as introduced in the Make team sites.

• Revamps style of handbook sidebar (including making menu collapsible) and in-content table of contents
• Removes search pane (with imminent switch to search widget) for handbooks
• Moves breadcrumbs into main content area for handbooks
• Moves #masthead out of, and before, #page
• Restyles masthead to have blue background
• Restyles slide down search pane to not match new masthead
• Introduces dropdown menu for masthead menu at smaller viewports (as used in WP-CLI Commands)
• Minor front page revamp of colors (more coming)

Fixes #2969,
See #2459, #3377.