An alternative way to make the code block stand out, could be to use backdrop-filter, which would adapt the background color. Like in the following variant:
.callout pre {
background-color: inherit;
backdrop-filter: brightness(120%);
}
Which in light mode does this to an example of mine:
Adjust the percentage according to your liking, but when above a 100 %, it kind of pops the color of the callouts a little, and I thought it looked a little nice.