Downstream typescript consumption fails: theme getter and setter diverge in type

[John Haugeland]

> jssm-viz@5.82.2 typescript
> tsc --build tsconfig.json

Error: node_modules/jssm/jssm.d.ts(629,9): error TS2380: 'get' and 'set' accessor must have the same type.
Error: node_modules/jssm/jssm.d.ts(630,9): error TS2380: 'get' and 'set' accessor must have the same type.
Error: Process completed with exit code 1.

Why?

  get themes(): FslTheme[] {
  set themes(to: FslTheme | FslTheme[]) {

Apparently those two types have to line up