Add subtle background to markdown headings.

[ryyHardy]

I think one thing that would be nice is to add a bit of a background to headings in the current markdown file (red-purple for h1 to h6 respectively). I think this is nice because it is much easier to tell what heading level you're at (especially with longer notes).

I attached a CSS snippet that adds the backgrounds in.

heading-backgrounds.css (feel free to adjust it)

Here is a screenshot of what the headings look like with the snippet enabled:

I think it would be cool to have this added to the theme in general. It wouldn't hurt.