feat: add Open edX Dark theme

[ihor-romaniuk]

Summary

• Add a new Open edX Dark theme — full dark token set (tokens/src/themes/dark/**) and committed generated CSS (styles/css/themes/dark/**), built via npm run build-tokens --all-themes
• Make hardcoded component styles theme-aware so they render correctly under any theme: --pgn-color-white/-black → bg-base/headings-base; theme-driven Modal popup arrow + scroll-shadow, DataTable loading overlay, Skeleton, and ProductTour shadows
• Lighten colored foregrounds for dark surfaces (links, outline buttons, IconButton, form validation, Breadcrumb/Stepper/Nav, SelectMenu link trigger), centralized in color.theme.default.*
• Distinguish Alert / variant background colors on dark surfaces
• Update doc examples that hardcoded light backgrounds (Modal/*.mdx, Menu/README.md) so popups/dialogs display correctly in dark
• Integrate the dark theme into the docs site (www) and example app, including a theme switcher
• lib/build-scss.js: build core + theme variants sequentially to avoid a theme-urls.json race condition

Context

Paragon themes are driven by design tokens, but only the light theme shipped — there was no usable dark theme. This PR adds one. While implementing it, many components turned out not to be theme-aware: they hardcoded white/black or pointed foreground tokens at the dark end of the color ramps, so they were invisible or low-contrast on dark surfaces. Those are fixed to consume theme tokens, with no change to the light theme (light-theme token files are separate and --pgn-color-bg-base/-headings-base resolve to the same white/black they did before). The result is a production-ready dark theme that existing consumers can opt into without affecting current light-theme behavior.

Manual Test plan

• Run npm start from the repo root, open the theme switcher, select Open edX Dark, and confirm the docs site renders in dark
• Spot-check key components in dark: Button (solid + outline variants), Hyperlink, Form (inputs + validation), Alert (all variants distinguishable), DataTable (incl. loading state — dark dim, not a white flash), Modal / ModalPopup (content + arrow on a dark surface), Menu / SelectMenu (link variant trigger legible), Chip, Breadcrumb, Stepper, Nav
• Switch back to light and confirm it is unchanged (no regression)
• Confirm npm run build-tokens and ./bin/paragon-scripts.js build-scss produce core, light, and dark and a complete theme-urls.json
• Confirm npx jest passes (977/977)
• Confirm the example app (example/) theme switcher toggles between light and dark

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