Wash
Two radial pools in the surface's colour, fixed under the page. The
<i> is the second pool at --hue-shift: 30. The host gets isolation: isolate. It follows the hue and the theme.
then move the hue slider
<div class="page">
<div class="wash" aria-hidden="true"><i></i></div>
…the regions…
</div>One per page, first child,
aria-hidden.