br-container does not get the correct margin when max-width is used for container

[guyathomas]

Steps to repro

• set max-w-prose class on the same element that has container
• Then use br-container as a child
• You will notice that the margin is not calculated correctly.

[guyathomas]

Found that document.documentElement.style.setProperty('--current-screen', 'calc(min(100%,65ch) + var(--container-padding) * 2 )') will set it correct.

I'll consider a good implementation - but the difficult part is that this can change on different pages depending on the container max width