Add light mode with theme switcher - #1637

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/github-script@v7, actions/setup-node@v4, pnpm/action-setup@v4, webfactory/ssh-agent@v0.9.1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/