Earlier quoted context omitted.
That’s because the HTML code is server-side rendered (SSR) with data-theme="dark" hardcoded on the element, so on the initial page load the browser immediately renders with dark mode styles applied. After ± 500ms-600ms Nuxt’s JavaScript hydration kicks in (as this is a Nuxt app based on __NUXT__ at line 11,236), which detects your macOS system preference via prefers-color-scheme media query and updates data-theme to…
it shouldn't need to do this. Nuxt has a @nuxtjs/color-mode module which ensures that the correct colour scheme is applied before the browser starts rendering the html. if the site author is lurking anywhere - may I recommend https://color-mode.nuxtjs.org ?
You can also expect some big changes to the UI very soon