fix(framework): make theme-inverted sections take the alt colors

[DavidBabinec]

The bug

Color tokens with an alt (dark) value emit their alt scope under :root.theme-default .theme-inverted, but nothing ever puts theme-default on <html>. A section with theme-inverted or theme-always-alt never switched, on published pages or in the canvas.

The fix

The default branch of the theme scopes now keys on :root:not(.theme-alt), so it matches every page as rendered today with no extra markup. A runtime toggle stays one class: adding theme-alt flips the page and every inverted section back. Sites with no dark-mode token emit no theme CSS, so their output is unchanged.

Verification

New tests parse the generated CSS and check Element.matches on a real document, both from generateFrameworkRootCss and from a published page. They fail on main and pass here.

(fail) publishPage > a theme-inverted section on a published page takes the alt color values
BUILD_EXIT=0  LINT_EXIT=0  TEST_EXIT=0
 7125 pass
 0 fail