Prevent flash of light-color background when loading new page in dark mode

Problem/Motivation

It is visually distracting when a website in dark mode, with a black background, flashes white while navigating between pages.

Steps to reproduce

Proposed resolution

Bulma itself has rejected changing this but there are suggestions here: https://github.com/jgthms/bulma/issues/2919

Remaining tasks

User interface changes

API changes

Data model changes