diff --git a/apps/web/src/lib/diffRendering.ts b/apps/web/src/lib/diffRendering.ts
index 2add48cad5724..fb306c3a45b75 100644
--- a/apps/web/src/lib/diffRendering.ts
+++ b/apps/web/src/lib/diffRendering.ts
@@ -291,37 +291,37 @@ export const DIFF_SURFACE_THEME_UNSAFE_CSS = `
   --diffs-bg-buffer-override: color-mix(in srgb, var(--code-background) 90%, var(--code-foreground));
 
   --diffs-bg-addition-override: light-dark(
-    color-mix(in srgb, var(--code-background) 50%, var(--diff-addition)),
-    color-mix(in srgb, var(--code-background) 70%, var(--diff-addition))
+    color-mix(in srgb, var(--code-background) calc(50% - var(--appearance-contrast-boost, 0%) / 4), var(--diff-addition)),
+    color-mix(in srgb, var(--code-background) calc(70% - var(--appearance-contrast-boost, 0%) / 4), var(--diff-addition))
   );
   --diffs-bg-addition-number-override: light-dark(
-    color-mix(in srgb, var(--code-background) 35%, var(--diff-addition)),
-    color-mix(in srgb, var(--code-background) 60%, var(--diff-addition))
+    color-mix(in srgb, var(--code-background) calc(35% - var(--appearance-contrast-boost, 0%) / 4), var(--diff-addition)),
+    color-mix(in srgb, var(--code-background) calc(60% - var(--appearance-contrast-boost, 0%) / 4), var(--diff-addition))
   );
   --diffs-bg-addition-hover-override: color-mix(in srgb, var(--code-background) 85%, var(--diff-addition));
-  --diffs-bg-addition-emphasis-override: color-mix(
-    in srgb,
-    var(--code-background) 80%,
-    var(--diff-addition)
+  /* Pierre resolves emphasis on the host, before the diff's overrides can inherit. */
+  /* Light emphasis stays above t3-light's readable syntax luminance floor. */
+  --diffs-bg-addition-emphasis: light-dark(
+    rgb(from var(--diffs-addition-base) r g b / 15%),
+    rgb(from var(--diffs-addition-base) r g b / calc(20% + var(--appearance-contrast-boost, 0%) / 4))
   );
 
   --diffs-bg-deletion-override: light-dark(
-    color-mix(in srgb, var(--code-background) 50%, var(--diff-deletion)),
-    color-mix(in srgb, var(--code-background) 70%, var(--diff-deletion))
+    color-mix(in srgb, var(--code-background) calc(50% - var(--appearance-contrast-boost, 0%) / 4), var(--diff-deletion)),
+    color-mix(in srgb, var(--code-background) calc(70% - var(--appearance-contrast-boost, 0%) / 4), var(--diff-deletion))
   );
   --diffs-bg-deletion-number-override: light-dark(
-    color-mix(in srgb, var(--code-background) 35%, var(--diff-deletion)),
-    color-mix(in srgb, var(--code-background) 60%, var(--diff-deletion))
+    color-mix(in srgb, var(--code-background) calc(35% - var(--appearance-contrast-boost, 0%) / 4), var(--diff-deletion)),
+    color-mix(in srgb, var(--code-background) calc(60% - var(--appearance-contrast-boost, 0%) / 4), var(--diff-deletion))
   );
   --diffs-bg-deletion-hover-override: color-mix(
     in srgb,
     var(--code-background) 85%,
     var(--diff-deletion)
   );
-  --diffs-bg-deletion-emphasis-override: color-mix(
-    in srgb,
-    var(--code-background) 80%,
-    var(--diff-deletion)
+  --diffs-bg-deletion-emphasis: light-dark(
+    rgb(from var(--diffs-deletion-base) r g b / 15%),
+    rgb(from var(--diffs-deletion-base) r g b / calc(20% + var(--appearance-contrast-boost, 0%) / 4))
   );
 
   background-color: var(--diffs-bg) !important;