Post editor shows persistent gray loading skeleton over header template-part; global styles injected incorrectly into editor iframe

While editing a WordPress page (Greenshift theme, block editor / "Blank canvas" page template with a custom header template part) the post editor consistently shows a full-width gray gradient bar in place of the page header/navigation area, right below the page title. The bar is persistent (not a hover tooltip — confirmed by moving the mouse away and re-screenshotting, the bar stayed in the exact same place). The editor also loads noticeably slowly (DOMContentLoaded ~2s, but the visual editor iframe with the header stays broken indefinitely). Devtools console shows a warning about global styles being injected into the editor iframe incorrectly. We rebuilt the header template part content from scratch (matching a known-working reference site on the same Greenshift theme/plugin stack) but the issue persists, suggesting the root cause is in how the theme/plugin injects global style custom properties into the block editor iframe for a page using a template that has a custom header template part, rather than in the page/template-part content itself.

Steps to reproduce
1. Use Greenshift theme with a page assigned the "blank" (Czyste płótno) page template.
2. Add a wp:template-part reference to a custom "header" template part inside the "blank" template.
3. Open any page using that template in the WordPress block editor (Gutenberg).
4. Observe the area where the header/navigation should render.

Expected
The header template part should render correctly (styled) inside the post editor preview, or at least not show a persistent unstyled gray loading placeholder.

Actual
A full-width gray gradient bar (looks like an unresolved loading skeleton) appears in place of the header/navigation and never resolves. It is not related to mouse hover (confirmed with two screenshots, one with cursor over the area and one with cursor moved away — bar unchanged in both).

Error output

properties-inline-css was added to the iframe incorrectly. Please use block.json or enqueue_block_assets to add styles to the iframe.
(source: global-styles-css-custom-properties-inline-css, block-editor.min.js)

Also present in console: Uncaught (in promise) Error: A listener indicated an asynchronous response, but the message channel closed before a response was received.

Context
Feature area: WordPress block editor (Gutenberg) + Greenshift theme template parts. Page template: blank (Czyste płótno). Template part: header (wp:template-part slug=header, theme=greenshift). Active plugins: greenshift-animation-and-page-builder-blocks, greenshiftgsap, greenshiftquery. The live front-end page renders correctly — this only affects the post editor preview.

Environment
App version: 10.5.5
Platform: Darwin 25.5.0 (arm64)