The most expensive sentence in web maintenance is “the brand color changed.” On most sites that sentence costs a week of find-and-replace. On this one it costs a single setting.
Every color, font, and spacing choice you make in theme settings compiles to a CSS custom property on the root element. Modules never hardcode a brand color; they reference the token.
Rebrand day stops being a project. Change the primary color once and buttons, links, icons, and accents update across every page — including pages built long after the change.
It also keeps the CSS payload small: one definition, referenced everywhere, cached once.
Theme settings compile to custom properties on :root — --primary-color, --heading-font, --border-radius and friends. Modules reference the tokens, never raw values, which is exactly the discipline the ITCSS settings layer exists to enforce.