Backport/revealjs/heading color (#12182)

[cderv]

Warning

This PR is a backport PR against v1.6 release

This port #12182 closing #12147 as this is fixing regression in patches for new revealjs update

We can merge if we do another patch release before next 1.7

• If we do this then don't forget to move the changelog 1.7 note in regression

quarto-cli/news/changelog-1.7.md

Lines 54 to 56 in 7f8cfd0

	## `revealjs` format
	- ([#12147](https://github.com/quarto-dev/quarto-cli/issues/12147)): `serif` and `simple` themes defaults back to have their heading color (`$presentation-heading-color`) to be the same as the body color (`$body-color`) as in Quarto 1.5.