sticky panel header needs background

[huwshimi]

Describe the bug

When using a sticky panel header p-panel__header is-sticky the header is transparent.

To Reproduce

Steps to reproduce the behavior:

1. Go to https://vanillaframework.io/docs/layouts/application#panels-example
2. Inspect the content header.
3. Add is-sticky to the p-panel__header
4. Scroll the page and see that the content is visible underneath.

Expected behavior

The content should not be visible behind the header.

Screenshots

Desktop (please complete the following information):

• OS: macOs
• Browser Chrome
• Version 119.0.6045.123