That’s because the HTML code is server-side rendered (SSR) with data-theme="dark" hardcoded on the <html> 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 "light".

This is much better than dark mode users getting flashbanged.

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 ?

I am still checking the post, and I've already added color mode 1-2 days ago.

You can also expect some big changes to the UI very soon