I am a bit of a rube so can't say for sure. I tried looking in hexbear-collectives's lemmy-ui repo but could not find where the colour for it is defined even though it is clearly there in the final CSS.
In the meanwhile until it is fixed you can use a browser extension that lets you modify CSS. On Firefox you can use Stylus for this. From there you can define a rule for https://hexbear.net with this CSS:
.featured-posts {
background: #ebebeb;
}
Then it should look like this: