Layout system should recognise ThemeProvider as valid wrapper round layout components

## Feature Request

### Description of Problem:
...what *problem* are you trying to solve that the project doesn't currently solve?

...please resist the temptation to describe your request in terms of a solution. Job Story form ("When [triggering condition], I want to [motivation/goal], so I can [outcome].") can help ensure you're expressing a problem statement.

### Potential Solutions:
...clearly and concisely describe what you want to happen. Add any considered drawbacks.

... if you've considered alternatives, clearly and concisely describe those too.