[bug] <rh-subnav> dark themes aren't supported, causing accessibility issues

[markcaron]

Describe the bug

Dark mode theming from parent element isn't supported in <rh-subnav>, which causes accessibility issues too.

Also related: document its support: #1479

Screenshots

Steps to reproduce

1. Go to Subnav's context demo
2. Switch to dark theme
3. See brokenness

Expected behaviour

Need to add dark mode theming support based on parent element's context.

Element HTML

Operating System (OS)

macOS

Browser

Chrome

Additional browser or assistive technology info

No response

[zeroedin]

You can change <rh-subnav color-palette="darkest"> and it works however it will not respond to its parent context as it defaults to lightest.