feat(ui): add persistent Dark Mode and Light Mode theme toggle

[vjkumar2756]

Fixed #3210

Summary

This PR adds full Dark Mode & Light Mode theme support to the Kubeflow Hub UI. It introduces a persistent theme switcher in the top navigation header bar, respects OS system preferences (prefers-color-scheme), and applies a high-contrast Midnight Slate Dark Design System across all UI components.

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What Changed?

1. New Theme Context (ThemeContext.tsx)

• Created CustomThemeProvider managing theme states (light, dark, system).
• Added localStorage persistence under key kubeflow_theme_mode.
• Added dynamic listener for system color scheme changes (matchMedia('(prefers-color-scheme: dark)')).
• Toggles .dark-theme and .pf-v6-theme-dark CSS classes on document.documentElement.

2. Navigation Bar Theme Switcher (NavBar.tsx)

• Added a Sun / Moon Toggle Button inside the top navigation masthead next to the user menu.
• Dynamically updates icon and tooltip based on active theme mode.

3. Application Root (App.tsx)

• Wrapped the application hierarchy with <CustomThemeProvider>.

4. Custom Dark Theme Styling (app.css)

• Midnight Slate Color Palette: Main background (#0b0f19), surface cards (#111827), borders (#1e293b).
• High Contrast Typography: Headings (#f8fafc), body text (#e2e8f0), links & accents (#38bdf8).
• Targeted Component Styling:
  • Dropdowns & Namespace Selectors (bella-namespace, user@example.com, Keyword filter).
  • Table headers, sort indicators, and table rows with hover highlights.
  • Bottom pagination toolbar background and control buttons.
  • Form controls, search inputs, pills, and chip tags.

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User Experience & Key Benefits

• Eye Strain Reduction: Significantly improves usability for ML engineers working in low-light environments.
• Instant 1-Click Toggle: Switch themes seamlessly from anywhere in the app without page reload.
• Zero Breaking Changes: Maintains 100% backward compatibility with Light Mode.

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Testing & Verification

• TypeScript Type Check: Passed (npm run test:type-check) — 0 errors found.
• Dev Server Build: Webpack dev server compiles cleanly without warnings or errors.
• Manual Verification: Tested theme switching, page reload persistence, namespace dropdown contrast, and pagination toolbar styling on local standalone deployment (http://localhost:9000).

[google-oss-prow]

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[vjkumar2756]

@pboyd
could you please review this PR when you get a chance.