Jenkins and plugins versions report

Paste the output here

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
2. Select Dark Theme
3. 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?