Theme styling (named themes + CSS custom properties)

[eduardozf]

Context

CONTEXT.md defines Theme Styling through SVG classes and CSS custom properties (not .wire syntax). The renderer already emits stable classes/metadata; expose theming on top.

Scope

• Define a small set of CSS custom properties (stroke, fill, label color, junction color, background).
• Optionally ship a dark theme stylesheet alongside the default standalone styles.
• A render option to select/omit the embedded theme.
• Snapshot tests for themed output.

Acceptance

• Output is restyleable via documented CSS variables without changing .wire source.
• Document the variable contract.