Re: line thickness

Thanks to Hammer for getting my attempt to work.

I did not understand your c) question (did not give the link you quote) 
and did not yet investigate a)

Let me answer to b)

> b) is it possible to parameter the same for horizontal lines (Hrule) ?

In the preamble (with the help of 
https://texmacs.github.io/notes/docs/macro-editor.html)

<assign|strike-through|<macro|body|<quasi|<datoms|<macro|x|<with|color|<unquote|<value|color>>|<repeat|<arg|x>|<resize|<tiny|->|<plus|0.6667l|0.3333r>||<plus|0.3333l|0.6667r>|>>>>|<arg|body>>>>>