feat(ui-shell): support theming

[metonym]

Closes #2457

PR that applies theming to the UI Shell.

TODO:

• Need to ensure all subcomponents (side navigation, panel are actually themed)
• Menu/navigation should have appropriate selected styles
• Use type style tokens if possible
• Currently, some selectors need higher specificity since v10 bundles with a subset of UI Shell styles. Can this be avoided? Should new classes be defined?

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