[Dashboard] Add dark mode support to the web dashboard

[rjwalters]

Feature Request

The SkyPilot dashboard's API server web GUI currently only supports a light theme. The Tailwind CSS configuration already has darkMode: ['class'] enabled and CSS variables defined for both light and dark modes in globals.css, but the dark mode is never activated — there's no theme toggle, no provider, and most components use hardcoded light-only color classes.

Live Demo

Try it here: http://rjwalters.info/skypilot/dashboard/

Click the theme toggle icon (Sun/Moon/Monitor) in the top-right navbar to switch between Light, Dark, and System modes.

Proposal

Add full dark mode support with a Light/Dark/System toggle in the top navbar, using next-themes for theme management. This involves:

1. Theme infrastructure — next-themes ThemeProvider + ThemeToggle component
2. Brand color adaptation — Convert sky-blue/sky-blue-bright to CSS variables that adapt between modes
3. Dark logo variant — Separate SVG with lighter fills for dark backgrounds
4. Component updates — Add dark: Tailwind variants across all ~30 component files
5. Global CSS — Dark mode styles for Shepherd.js tour, charts, CodeMirror, loading skeletons

Implementation: https://github.com/rjwalters/skypilot/tree/feature/dashboard-dark-mode
PR: #8938

Motivation

Dark mode reduces eye strain, improves accessibility, and is a standard expectation for modern web dashboards. The existing codebase already has partial dark mode infrastructure (CSS variables, Tailwind config), making this a natural completion of that work.