fix: default HR styling from background-color to border-top

[nathanlesage]

Hi all,

I propose to change the way horizontal rules are styled in HTML exports from applying a background-color to using border-top. This change does not affect how they are displayed; so all exports will still look like before.

However, this way, horizontal rules will be printed out if someone decides to save a document exported to HTML to PDF or print it.

Why?

I just had a user mentioning that, if you use a horizontal rule in a document, export it to HTML and then print it, the horizontal rule doesn't show up (see Zettlr/Zettlr#5562). After some fiddling around, I found that in the styles section, Pandoc applies a background instead of a border color. By default, every browser will not print backgrounds to save toner/ink, which applies even if someone wants to save down a PDF file.

In my specific case, it affects the way Zettlr's "Simple PDF" exporter works (which literally exports to HTML and then "prints" it using Chromium's HTML-to-PDF converter), but I believe that changing this will yield benefits for anyone using Pandoc to export to HTML. In addition, using a border color rather than a background color brings horizontal rule styling closer to how these elements are styled by default in a browser (using borders, not backgrounds).

Caveats

This PR does not affect the way horizontal rules are displayed, only how browsers treat them during PDF export/printing.

However, I want to note that this PR would necessitate minimal changes to two other open PRs:

• HTML template CSS: add dark mode with invert for code #7226 would need to change the way the background color is applied in dark mode
• Same for HTML template CSS: add dark mode with --highlight-style-dark #7131

[jgm]

Good idea.