Fix background color of impersonate button

What does this MR do and why?

Small regression was introduced when the background was removed in 581191-Fix in user_menu.vue (Delete CSS class) (!213854 - merged) • Andy Bonta • 18.7.

This change adds back background color with some adjustments to shape to accomdate avatars with a transparent background.

References

Closes #582979 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Impersonate a user
2. Try both light and dark mode

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