fix: white background for lightmode

[SethFalco]

Changes the light mode background to #FFFFFF.

The motivation is that we need to support HTML and PDF, and PDF is probably the more important of the two. Unfortunately, the PDF looks ugly with a non-white background as the background only extends to the content/body, not the remainder of the last page.

Ideally, I should've been able to use a media query like:

@media (print) {
  body {
    --bg-color: #FFFFFF;
  }
}

However, it seems the renderer doesn't use print media queries, so this was ignored. When or if this is supported, it'd be much better to change the background color back to #F9F9F9 and put the media query back.

Related

• Fix background color on PDF export #22