In Quarto we are trying to be clever about the YAML and so you can also put it at the first level and it would work. We are matching the name really and not just the position
Also, if you did not already, you should use the latest version of RStudio IDE (or preview or daily version if you want). In Quarto document, there is now YAML intelligence which means that your YAML will be checked for error in the IDE (and by Quarto) but you'll get also autocompletion for YAML field. So the hihglight-style field would appear if valid to put it there.