This is really great, thanks @Angel !
My flavor (uses rgba instead of background-color to allow for opacity adjustment):
.cm-s-obsidian span.cm-formatting-highlight,
.cm-s-obsidian span.cm-highlight {
background: rgba(255, 255, 0, 0.15);
color: rgba(255, 255, 120, 0.8);
}
.markdown-preview-view mark {
background: rgba(255, 255, 0, 0.15);
color: rgba(255, 255, 120, 0.8);
}