CSS revision to support fallback typefaces

[tiffehr]

Prior CoPilot feedback:

The fallback selectors cover header, footer, h1–h4, .button, .main-footer, which matches the elements in the corresponding sofia-pro rule at line 56. However, sofia-pro is also applied in several other rules (.hub .page-intro .sub-nav, .hub .upcoming-event div, .hub .previous-event h3/.when-where, .hub table.time-blocks, .page-divider, etc. — see lines 134, 143, 170, 192, 537, 550, 624, 628, 653). Those elements are not covered by the fallback selectors and will fall through to the browser's generic sans-serif rather than the explicit "Helvetica Neue", Arial, sans-serif stack when Typekit fails. For visual consistency, consider extending the fallback rules to cover all sofia-pro usages, or using a single html.wf-loading, html.wf-inactive, html.fonts-fallback-locked selector that redefines a CSS custom property.

Originally posted by @Copilot in #424 (comment)

[tiffehr]

Made irrelevant by #429