Custom theme options in existing tab for Pressbooks.

web.php

<?php

add_action( 'pb_theme_options_web_add_settings_fields', 'add_custom_web_settings', 10, 2 );
add_filter( 'pb_theme_options_web_booleans', 'get_custom_booleans' );

/**
 * Add the setting itself.
 *
 * @param string $_page The settings identifier, e.g. pressbooks_theme_options_web
 * @param string $_section The settings section identifier, e.g. web_options_section
 */
function add_custom_web_settings( $_page, $_section ) {
	add_settings_field(
		'custom_web_setting',
		__( 'Custom Web Setting', 'pressbooks' ),
		'render_custom_web_setting',
		$_page,
		$_section,
		array(
			__( 'Display a custom web setting', 'pressbooks' )
		)
	);
}

/**
 * Call the render function for a checkbox from the \Pressbooks\Options class.
 *
 * @param array $args
 */
function render_custom_web_setting( $args ) {
	$options = get_option( 'pressbooks_theme_options_web', \Pressbooks\Modules\ThemeOptions\WebOptions::getDefaults() );
	\Pressbooks\Options::renderCheckbox( 'custom_web_setting', 'pressbooks_theme_options_web', 'custom_web_setting', @$options['custom_web_setting'], $args[0] );
}

/**
 * Add our (boolean) option to the \Pressbooks\Options sanitization routines.
 *
 * @param array $settings
 */
function get_custom_booleans( $settings ) {
	$settings[] = 'custom_web_setting';
	return $settings;
}