Every Customizer setting is a theme modification stored with set_theme_mod(). Read one with knowx_mod( $id ): it returns the saved value, or the default that is declared on the Customizer field, so a default lives in one place only.
$width=knowx_mod('site_container_width'); // '1170px' until the owner changes it.
if ( knowx_is_truthy( knowx_mod('knowx_show_toc')) ) {
// Print the contents list.
}
Switches are stored as 0 or 1. Older sites may hold 'on', 'off', 'yes' or true, so test a switch with knowx_is_truthy() and never with a plain if ( $value ): the string 'off' is truthy in PHP.
Settings are registered in inc/Customizer_Settings/Fields/*_Fields.php, one file for each area. To add one, follow Adding a setting, a pattern or a hook.
The table lists all registered settings, grouped by where the owner finds them under Appearance > Customize. For what each one does for the site owner, see the Customizing pages.
Page Background Color is WordPress’ own background_color setting, relabelled by the Custom_Background component, so read it with get_background_color().
Colour settings that are empty by default (link, text, heading and content background) are derived from the theme colour in CSS, so leaving them empty keeps a readable result in light and dark mode. Typography settings are stored as one array each (family, size, weight, line height, letter spacing, transform and colour); an empty value falls back to the CSS in _custom-properties.css.
