[Jellby]

The HR is basically a rectangle with a border. Default colours, as well as thickness of the rectangle and border settings my vary between renderers. You could try this (untested):

hr.sectionbreak {
  width: 10%;
  margin: 1em 45%;
  height: 0; /* invisible rectangle */
  border-width: 0 0 1px 0; /* only bottom border */
  border-style: solid; /* choose other styles if you want other than a continuous line */
}

I believe by default the border will be rendered in the text colour (black normally, white in "night mode", etc.), so it's better not to touch it, but if you want to specify the rule colour, use border-color.