Custom CSS text added to style attribute of empty div

### Jenkins and plugins versions report

Environment

```text
Paste the output here
```

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

Rocky Linux 9.3 for the controller

### Reproduction steps

1. Add some css styling text to the "Custom CSS" field of the "Login Page Theme" section.
2. Bring up the login page.

### Expected Results

The added css styling text would apply to most elements of the login page. (E.g., it could be used in a style attribute of the login page's body element.)

### Actual Results

The added css styling text was in a style attribute of an empty div element (a div just before the for the logo).

### Anything else?

My workaround: add a ... section to the "Head" field of the "Login Page Theme" section.

### Are you interested in contributing a fix?

_No response_

Start with the Login Page Theme's Custom CSS handling and the rendered login page markup, comparing the configured text with the empty div before the logo. Done means the custom CSS is applied to the intended login-page elements rather than placed in the style attribute of an empty div.

Written by the indexing model from the issue text.