Fitting older Icarus Template with Updated Syntax Highlighting

I am trying to update Icarus theme (which had built in syntax highlighting) with GoldMark.

When I generate a syntax.css and put it in either the theme folder or a newly created /static/css folder (never needed before when using the theme) - I do not see a header loading the CSS file in the generated site (but I may be misunderstanding how this is loaded).

I tried replacing themes/hugo-icarus-theme/static/css/monokai.css with the generated syntax.css.

My primary symptom is that code fences appear unusual to me. I am including a screenshot.

I noticed Tory Anderson graciously updated it in July 2020 and I have cutover the updates.

I would be grateful for any and all tips on how to cleanly make sure only the new syntax highlighting is in play.

Without knowing your theme: highlight with line numbers creates probably a table and you probably have cellpadding somewhere defined. I would check the code that is generated and set it to 0 for table.highlight if that is the case. You could also set the background color for this section to that of the other parts so the white parts disappear into the background.