fix(theme): support global-header above-sidebar position

[logonoff]

Hey, I just made a Pull Request!

after:

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or Updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)

[rhdh-gh-app]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@red-hat-developer-hub/backstage-plugin-theme	workspaces/theme/plugins/theme	patch	v0.8.1

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[ciiay]

Thanks for the fix @logonoff 👍
Have you tested this pr in RHDH with multiple headers such as

• 2 headers above sidebar
• 2 headers above main content
• 1 header above sidebar + 1 header above main content

I also noticed that the top right border radius looks smaller than others in your screenshot, and the scrollbar is outside of the main content.

[logonoff]

Thanks for the fix @logonoff 👍 Have you tested this pr in RHDH with multiple headers such as

* 2 headers above sidebar

* 2 headers above main content

* 1 header above sidebar + 1 header above main content

I also noticed that the top right border radius looks smaller than others in your screenshot, and the scrollbar is outside of the main content.

two headers above sidebar + two headers above main content

one above sidebar + one above main content

two above sidebar

two above main content

[ciiay]

/lgtm

Thank you for the PR 🚀