Use different shade for code snippet background

[nitodeco]

Code snippets currently share their background color with the page background. This makes reading the package description a bit harder than it needs to IMO, especially in lightmode, since codeblocks aren't that visually distinct from any other text.

I propose that codeblocks get the same treatment as the command snippets, with a different background shade to elevate it from the rest of the page:

Let me know what you think and perhaps what the reasoning is for using identical background colors for code and the rest of the page.

[shuuji3]

closed by #973