Describe the bug

When starting with a fresh site and replacing the default header template part with Header -- Dark, unexpected horizontal padding is added to the header block itself.

To Reproduce
Steps to reproduce the behavior:

1. Create a new site with TT2;
2. Highlight the Header block;
3. Click Replace on the book settings popover;
4. Select Header -- Dark.

Verify that some horizontal padding has been added to the header itself.

Expected behavior

The header should span across the page, with no outer padding.

Screenshots

Editor	Live site

Additional context

This doesn't happen when:

• adding the template part by itself;
• adding a new header and clicking replace.

It only happens within the default Index template.