This may not solve the problem necessary because there may be other rules in your theme that have higher specificity and will overrule your selectors but
You need to combine the selector together, like so:
.postid-6238.inside-article {
background-color: #393939;
}
That selector will choose for all elements that have BOTH the classes .postid-6238 and .inside-article in them.
If you have them separated:
I’ve seen themes that will assign an id to the post-id instead of a class