--show-syntax-themes preview does not match actual rendering

delta --show-syntax-themes --dark is useful for browsing syntax themes, but the preview is misleading.

Selecting a syntax theme from:
```
delta --show-syntax-themes --dark
```

suggests a certain visual result, but using the same theme in normal delta usage:

```
delta --syntax-theme=""
```

can produce noticeably different output, including differences in diff backgrounds and empty-line rendering.

As a result, it is unclear what exactly the preview represents.

From a user perspective, --show-syntax-themes strongly suggests:

“this is how delta will look with that theme”

but actual rendering may differ enough to make theme selection confusing.

Possible improvements:

* clarify in help/docs that --show-syntax-themes previews syntax highlighting only, not full delta rendering
* make the preview path match normal rendering more closely
* document which rendering settings are intentionally different in preview mode

Start by comparing the --show-syntax-themes --dark preview path with normal delta --syntax-theme="" rendering, focusing on diff backgrounds and empty-line rendering. Determine whether the preview should match normal output or be explicitly described as syntax-only, then verify that the behavior or help/docs makes the distinction clear.

Written by the indexing model from the issue text.