[DYN-6370] Restore UI change in group context menu

[ivaylo-matov]

Purpose

This PR addresses DYN-6370 where some of the items in the group context menu have darker background that the rest.

Issue :
The inconsistency is there because the Package Manager context menus (which should be dark grey) share the same context menu item style as the group, connector, connector pin, and 3D watch node context menus (which should be light grey).

Fix :
I've created a separate style that applies a darker background color when IsMouseOver = False. It can be reused for future dark grey context menus.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated
• This PR contains no files larger than 50 MB

Release Notes

All items in the group, connector, connector pin and 3D watch node context menus now have consistent light grey backgrounds.

Reviewers

@QilongTang
@reddyashish

FYIs

@dnenov
@achintyabhat

[github-actions]

See the ticket for this pull request: https://jira.autodesk.com/browse/DYN-6370