[UI Shell docs] Header: Style tab

[laurenmrice]

Acceptance criteria

Tasks

• Update the Usage tab with the style content template.
• Update images to follow our image guidelines.
• Review / PR

[alina-jacob]

Pre-work inference

Figma reference file
Pre-work issue and comment.

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UI shell header

• Style tab
• Color: Add spec for divider - Divider | border | $border-subtle
• Sub-menu: Update sub-menu's Selected state to Open / Expanded.
  Should there be both 'Selected' and 'Open/Expanded' states?
• Sub-menu: Update sub-menu item's image to include item's Selected state as well.
• Typography: Add typography spec for sub-menu item - Sub-menu item | 14 / 0.875 | Regular / 400 | $body-compact-01