customize display of line numbers in code-block

[12rambau]

Fix #260

I don't like unclosed issues so I was scrolling in the repository to find small stuff that could be updated. I think some adjustment could be beneficial for the code-block directive when it come to line numbers.

The normal implementation place the numbers super close to the code and in the exact same color, making it a bit difficult to read.

With this PR I simply add a small padding to the line-number and dim the opacity so that they have less contrast than regular code (in short as in any good IDE).

Let me know what you think.

[choldgraf]

I think the design is nice, though it seems to not work on all code blocks? e.g. here: https://pydata-sphinx-theme--651.org.readthedocs.build/en/651/demo/kitchen-sink/paragraph-markup.html#code-with-sidebar

[12rambau]

I think the design is nice, though it seems to not work on all code blocks? e.g. here: https://pydata-sphinx-theme--651.org.readthedocs.build/en/651/demo/kitchen-sink/paragraph-markup.html#code-with-sidebar

there was a simple typo, should be solved now

[choldgraf]

LGTM, thanks!