Add widget theming docs to the skill

We should document how to use the theming tokens (--mw-*). Add a section to the skill covering: which tokens to reach for, when its ok to add your own component-specific token, how to connect/apply a theme and a note on the canvas/webgl pattern (copy chart/src/theme.ts as the starting point) since that one's not obvious without an example. Bonus if the skill can eventually ask a new widget author "is this canvas/webgl based?" and scaffold accordingly, but docs first.

Locate the skill documentation and read chart/src/theme.ts as the concrete canvas/WebGL example. Add guidance on choosing --mw-* tokens, creating component-specific tokens, applying themes, and the canvas/WebGL pattern; done means the skill covers these cases without implementing the optional scaffolding question.

Written by the indexing model from the issue text.