This commit fixes a few edge-cases. First, if you `prefers-color-scheme`
is dark, your background will no longer be dark in light mode.
Furthermore, switching from dark to system theme now works. It did not
before, as we queried our iframe for color-scheme instead of the parent
window (our iframe inherited the color-scheme from the parent document).
Finally, the navbar iframe has its color-scheme set separately, which
caused issues in a few cases.
76a137dabc