The oh-list-card is one of the special cases. In this case, the style property isn’t actually read so you can’t set css or css variables that way. In order to apply css to the header in the oh-list-card you have to use the more direct option of the stylesheet property which allows you to inject css directly into the page at the location of that element. So in this case you would want something like this:
- component: oh-list-card
config:
title: List Card
stylesheet: >
.card-header {
font-weight: bold;
}