Consolidate styles for `old_content_warning` and `notice warning` shortcode

[jaestevan]

Describe the bug

Would it make sense to consolidate CSS styles for old_content_warning and notice warning shortcode banners? So far they show in a completely different style and the result is not really nice.

To Reproduce

Steps to reproduce the behavior:

1. Activate old_content_warning
2. Navigate to a post that is older than the selected value
3. add a notice warning shortcode to the page
4. See the resulting rendering

Expected behavior

As these two show as banners on top of the articles, it would be nice if styles are more cohesive.

Screenshots

Desktop (please complete the following information):

• OS: Windows
• Browser: Chrome
• Version of the Theme: Last
• Version of Hugo: Last

Smartphone (please complete the following information):

• Device: iPhone 15
• OS: Last
• Browser Safari
• Version of the Theme: Last
• Version of Hugo: Last