Audit custom-theme and website color-mode VR scenarios

Is your feature request related to a problem? Please describe. Not every visual-regression test should be blindly wrapped by an outer light or dark provider. The dedicated tests/theme/** suite intentionally supplies custom, empty, nested, or component-specific themes, and the website dark-mode tests exercise document color mode rather than only Recharts theme context.

Describe the solution you'd like Audit all dedicated theme scenarios and special website theme/color-mode stories, including test-vr/tests/theme/**, test-vr/tests/www/Theming.*, and test-vr/tests/www/dark-mode.*.

For every scenario, decide and document one of the following:

• It should run through the ordinary legacy/light/dark matrix.
• It should run a specific subset using the structured variant suppression introduced by #7736.
• It is an explicit provider/color-mode test and remains deliberately self-configured outside the generic matrix.

Remove obsolete manual testTheme props and theme-only title suffixes where doing so does not erase a meaningful semantic distinction. Preserve tests that specifically compare emptyTheme, nested providers, custom themes, or website prefers-color-scheme behavior.

Describe alternatives you've considered Applying the generic outer provider to every theme test can mask the very provider precedence and compatibility behavior those tests are supposed to validate.

Skipping the entire specialized suite from theme work would leave unclear and undocumented coverage gaps.

Additional context Blocked by #7736. The key distinction is that legacy means no provider, while emptyTheme means a present provider on the new theme path.

Acceptance criteria:

• Each special-purpose spec has a deliberate, documented variant policy.
• The common opt-out mechanism is proven against representative test and test.describe scopes.
• Website color mode remains independently controlled and does not accidentally select a Recharts theme.
• No obsolete hand-wired theme test remains solely because the automatic harness was unavailable.