After learning a lot about style sheets from reading this thread, and by combining various ideas throughout the thread, and quite a bit of cut-and-try, I finally have some CSS-only settings that work for me^*, so here's what I have:

In Display Settings / Code Listing Settings I have two bits:

• Code Prefix: I've set it to "<font color="red"><i></i></font>"
• On-Site CSS Markup: I've added:
  

  pre.code tt.codetext
  { counter-reset: codeLines; }
  
  pre.code i::before
  { counter-increment: codeLines;
    content: counter(codeLines,decimal-leading-zero) ": ";
  }
  [download]

I'm pretty happy with the result, as I can still have my line numbers and no longer have the line numbers come through on cut-and-paste.

I originally had (cadged from Anonymous Monk in this post) div.codeblock in the counter reset list, but that had the unfortunate effect of making small codeblocks (ones without the download link) all have the same line number). (While I learned more about CSS, I didn't learn enough about know/explain why.)

Anyway, if people could try it on other browsers & such, I'd appreciate it. (I'll give MacOsX / safari & chrome a try when I get back to $work on Monday).

*: I'm using Microsoft Windows 8.1 and FireFox 35.0.1 as I'm testing today.

Update: By the way, I've tried to fix the line-wrap by replacing the content like so:

span.line-break { content: '' }
[download]

but it doesn't hide the line wrap marker. Considering that I'd often hand-edit to remove a few line numbers (when I only wanted a section of the OP), it's no big deal. If it becomes bothersome, I can always turn off line wrapping.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

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