Material Theme preview renders incorrectly in theme manager settings.

### Jenkins and plugins versions report

Environment

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### What Operating System are you using (both controller, and any agents involved in the problem)?

Any, all.

### Reproduction steps

1. Install Dark Theme
2 Install Material Theme
3. Select Dark Theme
4. Look at previews of Material Theme in Configure

### Expected Results

The Material theme preview images render correctly (i.e. have white background)

### Actual Results

Material theme renders with dark background despite setting white background.


### Anything else?

@timja do you have any pointers on this one?