notice: fix layout when content starts with heading

[andi-blafasl]

If you have a heading as the first line of content inside a Notice box, it will break the layout as you can see in the screenshot.

This is the code to reproduce the issue:

{{% notice note %}}

#### So verhalten Sie sich richtig:

- Beantworten Sie keine Spam-E-Mails
- Öffnen Sie keine Anhänge von unbekannten Absendern
- ....

{{% /notice %}}

Not sure, but the fix for the issue should already be inside theme.css

But it is ignored by the Browser and "#body h4" is used instead