Styling regression on border in latest 1.6

[cderv]

I noticed some changes locally when building with dev version and now it is in quarto.org
as #11283 was added to prerelease

Previous doc

Now with darker separator

So we should try mitigate that while keeping #11282 that #11283 dealt with

[cscheid]

it turns out that mix works backwards from how I thought it did: mix(a, b, 90%) means "90% of a, 10% of b". That's completely unintuitive to me, but it's an easy fix. I'll push a quick direct fix to main.

[cderv]

Thanks !