Get information about a given theme from the WordPress.org API.
If provided with a callback, will call that on succes with an object with theme details.
Otherwise, will return a promise.

@param {string}     themeId  The theme identifier.
@returns {Promise.<Object>}  A promise that returns a `theme` object
function( themeId ) {
		const query = {
			action: 'theme_information',
			// Return an `author` object containing `user_nicename` and `display_name` attrs.
			// This is for consistency with WP.com, which always returns the display name as `author`.
			'request[fields][extended_author]': true,
			'request[slug]': themeId,
		};
		return superagent
			.get( WPORG_THEMES_ENDPOINT )
			.set( 'Accept', 'application/json' )
			.query( query )
			.then( ( { body } ) => body );
	}