Theme manager theme selection makes some themes look different in depending on the current theme

### Jenkins and plugins versions report

2.576 + latest plugins

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

irrelevant

Firefox 153

### Reproduction steps

1. Install a bunch of themes
2. Toggle back and forth between some themes on the UI

### Expected Results

The theme preview images stay the same

### Actual Results

They don't for:

* Material -- appears as "light" or "dark" theme based on what's currently selected
* Solarized -- borders appear differently

### Anything else?

These two screenshots on which I deleted the background make this pretty apparent when toggling between the two:

### Are you interested in contributing a fix?

no

Start by reproducing the issue in the theme manager UI with Material and Solarized themes, toggling between themes as described. Compare the preview images before and after each toggle; done when both previews remain visually unchanged regardless of the currently selected theme.

Written by the indexing model from the issue text.