I am as far from a CSS expert as one can get, so with that being said, this gave me the yellow I was looking for. If you want it a bit fainter, just reduce the 0.40 to something less until it suits your eyes. If you want the shade stronger, then of course adjust the number upward.
}
.cm-s-obsidian span.cm-formatting-highlight, .cm-s-obsidian span.cm-highlight {
background-color: rgba(255, 242, 0, 0.40);
}