First line formatting in the code block using fences

[adanielov]

This is same issue as in #84

Unfortunately, the Highlight plugin mentioned in #84 as a workaround does not meet our needs.
Hoping to get the root cause fixed.

padding: .2rem .4rem; in css-compiled/theme.css seems to be creating the trouble. But where does it come from?

It comes from _typography.scss, which has the rule of padding: .2rem .4rem. Editing the scss won't work unless you install additional dependencies to rebuild it, so it's best to edit the css-compiled/theme.css and change this line to padding: .2rem .0rem to fix the first-lin code indentation problem.