Currently, One Light sets the background color of hr tags in markdown preview panes as the same as that of the pane. So, they are not visible in the preview (as can be seen in the screen-shot of the linked issue). This is most definitely not the desired behaviour...
A simple change to the css rule for hr would be enough for fixing this issue.
Carried over atom/markdown-preview#267.
Currently, One Light sets the background color of hr tags in markdown preview panes as the same as that of the pane. So, they are not visible in the preview (as can be seen in the screen-shot of the linked issue). This is most definitely not the desired behaviour...
A simple change to the css rule for hr would be enough for fixing this issue.