sf-lwc-theming: Custom Themes for LWC

Create and apply custom themes that extend SLDS 2 for branded Salesforce experiences. Build theme systems that support multiple brands and component-level overrides — all without breaking the design system. Light mode is the default; dark mode is an optional enhancement when explicitly requested.

Core Principles

1. Light mode first — all themes default to light mode; dark mode is opt-in
2. Extend, never override — add brand tokens alongside SLDS hooks, don't reassign them
3. Semantic naming — brand tokens describe purpose, not color (--brand-primary, not --brand-blue)
4. Theme at the boundary — apply themes at the :host level, not deep in component CSS
5. Portable — themes work in internal Lightning, Experience Cloud, and mobile

Theme Architecture

┌─────────────────────────────────────────────┐
│  Brand Theme Layer (your custom tokens)     │  --brand-*, --app-*