Figma link: https://www.figma.com/design/qRn3S225WwGHIdgGZX0LTC/Unified-Theme--Design-Tokens--Styles---Specs?node-id=10826-11738&m=dev

Accordion needs the following updates to support glass theme:

• opt-in modifier for plain/no background

• opt-out modifier for not-plain/background

• automatic opt-in to plain styling when in a glass container & when glass theme is applied, unless opt-out is specified

Need design review of whether nested/expanded accordion backgrounds need to be plain, but minimally the outermost Accordion list needs to be plain.

Jira Issue: PF-3522