Scope global styles to the #root container

[bclinkinbeard]

User-Facing Changes

None

Description

Scopes the global styles to the #root container so embedding Studio within another app doesn't affect things outside its parent node.

[github-actions[bot]]

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[defunctzombie]

Closing this out in favor of https://github.com/foxglove/studio/pull/3802. We should be able to land 3802 once we remove fluentui. I don't have an exact timeline for that removal because we want to give folks time to migrate off the 3d (legacy) panel themselves.