Fix DockPanel Tab Styles

[GordonSmith]

Tweaked styles to re-use fluentui styles

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• This change is a bug fix (non-breaking change which fixes an issue).
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• This change alters the query API (existing queries will have to be recompiled)

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Testing:

[GordonSmith]

Not technically needed for the Tab fix - but makes more consistent with the fluent ui version