DropdownMenu v2 style tweaks

[ciampo]

What?

Fixes #50910

Tweak the design of the new DropdownMenu component following the feedback from #50910

Why?

We want our components to look good and consistent with the rest of the editor UI

How?

• Changed separator to go full width
• Changed hover / focus / open styles to have a light gray background

Testing Instructions

• Open the component's Storybook example
• Make sure that the tweaks match the feedback from DropdownMenu: Design feedback #50910

Screenshots or screencast

trunk	This PR

[ciampo]

My personal notes:

• currently hover, focus and open styles (ie. the style of a menu item when its corresponding submenu is open) are currently visually represented the same way
• with the new styles, there's no usage in the components, of the main accent color (defaulting to blueberry)

[github-actions]

Flaky tests detected in cee5a4d.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/5081049790
📝 Reported issues:

• [Flaky Test] No Behaviors should be the default as defined in the core theme.json #50923 in /test/e2e/specs/editor/various/behaviors.spec.js

[chad1008]

Changes look good to me and the new styles are looking great in Storybook. I agree on the full-width separators, I think it looks a lot nicer that way, personally. I'm sure you'll want input from others who offered feedback initially but I say 🚀 🚢

[jameskoster]

I will change the hover status to gray, although we'd need to also consider:

• focus state
• "expanded" state (ie. the state that a subtrigger is in, when its corresponding submenu is open)

If we're committed to the radix-style color scale there are guides for these details. But:

• I don't know that we are committed?
• The current colors do not map adequately (we only have 8 vs Radix' 12).

It probably doesn't need to hold up this PR, but it feels like something that we need to make a decision on sooner rather than later.

[ciampo]

it feels like something that we need to make a decision on sooner rather than later.

Absolutely. IMO, the current color config that we have doesn't feel comprehensive and clear to apply. We often end up having to interpret it, fill in gaps and/or making exceptions because the color combinations don't quite work out.

This definitely feels like a bigger task than the DropdownMenu. We can probably merge the PR as-is if that works for you, and revisit the component once we have more clear guidelines ?

Also flagging #50971 which is partially related to this topic

[ciampo]

As discussed in person with @jameskoster , I will go ahead and merge this PR, and later work on updating the styles of this component to match the current look&feel of the legacy DropdownMenu component.