Improve downloads website styling with dark mode and wider layout

Extend sites/eclipse/page.css with a CSS-variable palette (Eclipse-blue
accent), automatic dark mode via prefers-color-scheme, Inter and
JetBrains Mono typography, card-style tables with larger headers,
card-shaped details/accordions and a gradient-stripe popup dialog.

HTML markup, JSON schema and the build pipeline are untouched.

Fixes #3812

Author: vogella