* Consider both `selectedChangesetStatus` and `changesetStatus` states when deciding to disable.
* Factor out common logic into `canSwitchTheme` function on `ThemesPanel`.
* Keep Live Preview and Install buttons disabled in Themes controls and detail overlays when appropriate.
Props westonruter, dlh.
Amends [41788].
See #42126, #37661, #39896.
Fixes#42406.
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