Fix remaining invalid variable instantiations

[Mrmaxmeier]

Hi,

I ran into a parser failure after I tried importing marx.css:

 ⨯ ./node_modules/marx-css/css/marx.css:659:25
Parsing css source code failed
  657 |   border: 0;
  658 |   margin: 0;
> 659 |   padding: var(--sm-pad 0);
      |                         ^
  660 | }
  661 | legend {
  662 |   border-bottom: var(--border);

Unexpected token Number { has_sign: false, value: 0.0, int_value: Some(0) } at [project]/node_modules/marx-css/css/marx.css:658:24

After fixing these, the parser (Lightning CSS via Turbopack / Next.js) doesn't complain any more. See similar PRs (with incomplete fixes): #70 #63

I've included updates to the marx.css and marx.min.css files in this PR. Let me know if that works for you or if you'd prefer updating those yourself to reduce merge conflicts.

Thanks!