Theme-aware tokens for graphs

😯 Problem to Solve

The PDV app in SLE is going to add a Plotly graph to their app. It would be good for Nimble to help them style it with theme-aware colors/fonts/spacing/line thicknesses.

💁 Proposed Solution

Ideas:

1. Provide dedicated theme-aware tokens for the graph
2. Update our documentation for existing generic theme-aware tokens to describe how they apply to graphs
3. Ship a style sheet that applies tokens to a plotly graph
4. Provide an example of applying tokens to a plotly graph
5. A dedicated nimble-graph component that wraps plotly in a FAST template and applies the tokens automatically
   • in past discussions we've leaned away from this approach because the ROI seems low...it would take a lot of work to expose all the APIs for the graph in a custom element and apps tend to want to do a lot of customization of graphs

## 📋 Tasks
- [ ] #787
- [ ] Implement tokens

Start with task #787 and the Plotly graph use case described in the issue. Resolve whether the scope is tokens, documentation, a stylesheet, an example, or a nimble-graph component; completion is not defined until one approach and its acceptance criteria are selected.