feat: allow transparent background via config or flag

src/ui/App.tsx

@@ -138,7 +138,10 @@ export function App({
     () => availableThemes(bootstrap.customTheme),
     [bootstrap.customTheme],
   );
-  const baseTheme = resolveTheme(themeId, detectedThemeMode ?? null, bootstrap.customTheme);
+  const baseTheme = useMemo(
+    () => resolveTheme(themeId, detectedThemeMode ?? null, bootstrap.customTheme),
+    [themeId, detectedThemeMode, bootstrap.customTheme],
+  );
   const activeTheme = useMemo(
     () =>
       bootstrap.input.options.transparentBackground

src/ui/diff/pierre.test.ts

@@ -13,7 +13,7 @@ import { resolveSplitPaneWidths } from "./codeColumns";
 import { renderCodeOnlyPlannedRowText, renderDecoratedPlannedRowText } from "./renderRows";
 import { buildReviewRenderPlan } from "./reviewRenderPlan";
 import { measureTextWidth } from "../lib/text";
-import { resolveTheme } from "../themes";
+import { TRANSPARENT_BACKGROUND, resolveTheme, withTransparentBackground } from "../themes";
 
 function createDiffFile(): DiffFile {
   const metadata = parseDiffFromFile(
@@ -146,6 +146,28 @@ describe("Pierre diff rows", () => {
     ).toBe(true);
   });
 
+  test("keeps word-diff highlight backgrounds transparent in transparent mode", async () => {
+    const file = createDiffFile();
+    const theme = withTransparentBackground(resolveTheme("midnight", null));
+    const highlighted = await loadHighlightedDiff(file);
+    const rows = buildSplitRows(file, highlighted, theme);
+    const changedRow = rows.find(
+      (row) =>
+        row.type === "split-line" && row.left.kind === "deletion" && row.right.kind === "addition",
+    );
+
+    expect(changedRow).toBeDefined();
+    if (!changedRow || changedRow.type !== "split-line") {
+      throw new Error("Expected a split-line change row");
+    }
+
+    const removedWordSpan = changedRow.left.spans.find((span) => span.text.includes("41"));
+    const addedWordSpan = changedRow.right.spans.find((span) => span.text.includes("42"));
+
+    expect(removedWordSpan?.bg).toBe(TRANSPARENT_BACKGROUND);
+    expect(addedWordSpan?.bg).toBe(TRANSPARENT_BACKGROUND);
+  });
+
   test("builds stacked rows with separate deletion and addition lines", () => {
     const file = createDiffFile();
     const theme = resolveTheme("paper", null);

src/ui/diff/pierre.ts

@@ -11,7 +11,7 @@ import { formatHunkHeader } from "../../core/hunkHeader";
 import type { DiffFile } from "../../core/types";
 import { blendHex, hexColorDistance } from "../lib/color";
 import { sanitizeTerminalLine } from "../../lib/terminalText";
-import type { AppTheme } from "../themes";
+import { TRANSPARENT_BACKGROUND, type AppTheme } from "../themes";
 import { expandDiffTabs } from "./codeColumns";
 
 const PIERRE_THEME = {
@@ -238,6 +238,26 @@ function strengthenWordDiffBg(lineBg: string, signColor: string) {
   return strongestCandidate;
 }
 
+/** Return whether a theme color can safely participate in RGB distance and blend math. */
+function isHexThemeColor(color: string) {
+  return /^#[0-9a-f]{6}$/i.test(color);
+}
+
+/** Resolve one word-diff background without turning transparent surfaces into black blends. */
+function resolveWordDiffHighlightBg(contentBg: string, lineBg: string, signColor: string) {
+  if (contentBg === TRANSPARENT_BACKGROUND || lineBg === TRANSPARENT_BACKGROUND) {
+    return contentBg;
+  }
+
+  if (!isHexThemeColor(contentBg) || !isHexThemeColor(lineBg)) {
+    return contentBg;
+  }
+
+  return hexColorDistance(contentBg, lineBg) >= MIN_WORD_DIFF_BG_DISTANCE
+    ? contentBg
+    : strengthenWordDiffBg(lineBg, signColor);
+}
+
 /** Resolve the inline word-diff background, strengthening theme colors that are too subtle to see. */
 function wordDiffHighlightBg(kind: SplitLineCell["kind"], theme: AppTheme) {
   const cacheKey = [
@@ -251,14 +271,16 @@ function wordDiffHighlightBg(kind: SplitLineCell["kind"], theme: AppTheme) {
   ].join(":");
   let cached = wordDiffBackgroundCache.get(cacheKey);
   if (!cached) {
-    const addition =
-      hexColorDistance(theme.addedContentBg, theme.addedBg) >= MIN_WORD_DIFF_BG_DISTANCE
-        ? theme.addedContentBg
-        : strengthenWordDiffBg(theme.addedBg, theme.addedSignColor);
-    const deletion =
-      hexColorDistance(theme.removedContentBg, theme.removedBg) >= MIN_WORD_DIFF_BG_DISTANCE
-        ? theme.removedContentBg
-        : strengthenWordDiffBg(theme.removedBg, theme.removedSignColor);
+    const addition = resolveWordDiffHighlightBg(
+      theme.addedContentBg,
+      theme.addedBg,
+      theme.addedSignColor,
+    );
+    const deletion = resolveWordDiffHighlightBg(
+      theme.removedContentBg,
+      theme.removedBg,
+      theme.removedSignColor,
+    );
 
     cached = {
       addition,