[FIX] website, *: reset header items colors to default when transparent

*: web_editor

Users can choose one of their color combination to apply on the header.
If they then choose to make the header transparent over the content for
a specific page, the color of buttons & other components in the header
should be reset to the default website color combination's colors as the
header component are now appearing over the default content.

Part of https://github.com/odoo/odoo/pull/65139