Refactor styles.css onto a single set of light/dark theme tokens

Replaces ~50 scattered body.dark per-selector color overrides with
semantic CSS custom properties (--bg, --text, --border, --invert-bg,
etc.) defined once in :root and redefined once under body.dark, so any
new section inherits correct theming automatically.

Also fixes two existing dark-mode bugs this surfaced: .btn-primary had
no dark styling and was nearly invisible against a dark background,
and the footer logo's dark override was un-inverting it back toward
black on the footer's always-dark background.

Author: shahidghafoor00