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

CHANGELOG.md

@@ -5,6 +5,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## Draft
+- Add checkout style overrides for enhanced checkout theme. [#2721](https://github.com/bigcommerce/cornerstone/pull/2721)
 
 ## 6.21.0 (07-21-2026)
 - Display featured promotion callouts on product listing pages (product cards and list view) and the PDP, gated by the new `show_featured_promotions` theme setting and the `featured_promotions` context [#2695](https://github.com/bigcommerce/cornerstone/pull/2695)

assets/scss/optimized-checkout.scss

@@ -379,3 +379,57 @@ $optimizedCheckout-input--error-boxShadow: 0 0 3px transparentize(stencilColor("
     background-color: stencilColor("optimizedCheckout-loadingToaster-backgroundColor");
     color: stencilColor("optimizedCheckout-loadingToaster-textColor");
 }
+
+
+//
+// Styles for enhanced theme, which is applied when the
+// "Enhanced Checkout" option is enabled in the control panel.
+// -----------------------------------------------------------------------------
+
+.enhancedThemeV1 {
+    background-color: stencilColor("optimizedCheckout-header-backgroundColor");
+
+    .optimizedCheckout-form-checklist-checkbox:checked ~ .optimizedCheckout-form-label::before {
+        background-color: stencilColor("optimizedCheckout-formChecklist-color");
+        border-color: stencilColor("optimizedCheckout-formChecklist-color");
+    }
+
+    .optimizedCheckout-form-checklist-checkbox:checked ~ .optimizedCheckout-form-label::after {
+        background-color: stencilColor("optimizedCheckout-formField-backgroundColor");
+    }
+
+    .optimizedCheckout-form-checklist {
+        .optimizedCheckout-form-checklist-item {
+            border-color: stencilColor("optimizedCheckout-formChecklist-borderColor");
+        }
+
+        .optimizedCheckout-form-checklist-item--selected {
+            border-color: stencilColor("optimizedCheckout-formChecklist-color");
+            background-color: stencilColor("optimizedCheckout-formChecklist-backgroundColorSelected");
+        }
+    }
+
+    .optimizedCheckout-contentPrimary {
+        .optimizedCheckout-checkoutStep {
+            background-color: transparentize(stencilColor("optimizedCheckout-body-backgroundColor"), 0.5);
+
+            &.checkout-step--current {
+                background-color: stencilColor("optimizedCheckout-body-backgroundColor");
+            }
+
+            &:not(.checkout-step--current) .optimizedCheckout-buttonSecondary {
+                background-color: transparent;
+            }
+        }
+    }
+
+    .optimizedCheckout-orderSummary {
+        .optimizedCheckout-orderSummary-toggle {
+            color: stencilColor("optimizedCheckout-link-color");
+
+            svg {
+                fill: stencilColor("optimizedCheckout-link-color");
+            }
+        }
+    }
+}