feat(checkout): CHECKOUT-10198 apply new style overrides for enhancedTheme in checkout

[richapatel1510]

What?

Add new style overrides to support the new enhanced checkout theme UI in dark variants as well.
Works in along with checkout-js changes bigcommerce/checkout-js#3228

Requirements

• CHANGELOG.md entry added (required for code changes only)

Tickets / Documentation

Add links to any relevant tickets and documentation.

• CHECKOUT-10198

Screenshots (if appropriate)

BEFORE

BOLDTheme-BEFORE.mov

AFTER

BOLDTheme-AFTER.mov

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Note

Low Risk
Checkout-only SCSS scoped to .enhancedThemeV1 with no payment or auth logic changes; risk is limited to visual regressions on Enhanced Checkout when enabled.

Overview
Adds .enhancedThemeV1 rules in optimized-checkout.scss so Optimized Checkout can match the new Enhanced Checkout UI (including dark variants), paired with checkout-js changes.

When the control panel Enhanced Checkout option is on, checkout-js applies this wrapper class; Cornerstone now maps existing optimizedCheckout-* theme colors to checklist radios, step panel backgrounds (current vs inactive), secondary buttons on non-current steps, and order-summary toggle text/icon fill. Header background color drives the enhanced theme page background.

CHANGELOG draft entry documents the theme update.

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