I've had a look at Style Properties: Sidebar but can't see anything obvious in there to change things.
I've inspected the element & it looks like the colour for the post title is taken from .secondaryContent
But when I've changed that it's changed the colours elsewhere as well, which I don't want.
The post details are from body .muted, again when I change that it also changes in places I don't altered.
I've tried adding .sidebar .secondaryContent to extra.css but that doesn't change the new post sidebar.
I'm sure it must be something obvious but I'm missing it at the moment.