Themes: where the site colours come from

Colours, font and background are set by the active theme. It declares a set of variables and all the markup refers to them — which is why switching a theme repaints the whole site without touching a single template.

--color-primary  the main colour
--color-accent   the secondary one
--surface        card background
--surface-ink    text colour on it

The rule is simple: never write a colour as a literal in new markup. Do it once and that piece stops following the theme — and on a dark look the text disappears. That has happened more than once.