Fix Header Vertical overlay colors to use Ipsum palette slugs

[shail-mehta]

Description

The Header Vertical pattern set the navigation overlay to base / contrast, which are not registered in Ipsum’s palette (theme-1- theme-6). The open menu fell back to a plain white panel with black text and ignored the active color variation.

This updates the overlay to theme-6 (background) and theme-1 (text), matching the theme’s other inverted overlays (e.g. navigation overlay) so the menu tracks every color variation.

Screenshots

Before	After

Testing Instructions

1. Activate Ipsum; clear any customized Header template part if Header Vertical was already inserted (Site Editor → Header → Clear customizations), or re-insert the Header Vertical pattern and save.
2. On the front end, open the menu from Header Vertical.
3. Confirm the overlay uses theme colors (not a plain white sheet).
4. Switch to at least one other color variation (e.g. After Hours and the default) and confirm the overlay still contrasts correctly (theme-6 background / theme-1 text).
5. Confirm typography style variations are unaffected.

Use of AI Tools

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Co-authored-by: shail-mehta <shailu25@git.wordpress.org>

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