We recently removed Kirigami Headings' slight transparency, after concluding that it looked inappropriate to have Headings with lighter text than their textual content. `PlasmaExtras.Heading` does the same thing, so we should make the same change here, both based on the inherent merits of the change, and also to maintain consistency with Kirigami.
This also slightly improves matters for people who complain about poor text contrast throughout Plasma.