`UI.getTheme()` reports system UI theme on startup, instead of Sketch's UI theme
Let's say my system theme is dark, but my Sketch theme is light.

In my `onStartup()`:

```js
export const onStartup = async () => {
  log(`DEBUG: ${UI.getTheme()}`); // 'DEBUG: dark'
}
```

In some other action that occurs once the plugin has properly loaded:

```js
export const onAction = async () => {
  log(`DEBUG: ${UI.getTheme()}`); // 'DBEUG: light'
}
```

I can work around this for my current use case, but thought it was worth reporting.

Also, should there be a way to query System theme, Sketch theme, and Sketch canvas theme separately?