FIX: make word emphasis readable on light themes

src/diff/index.spec.ts

@@ -131,6 +131,15 @@ test("word emphasis pairs the most similar lines inside change blocks", () => {
   assert.match(rendered[0] ?? "", /\x1b\[48;2;148;62;70m\x1b\[1mline/);
 });
 
+test("word emphasis uses readable backgrounds with light syntax themes", () => {
+  setCodePreviewSettings({ ...codePreviewSettings, shikiTheme: "github-light" });
+  const diff = "-1 const value = oldValue;\n+1 const value = newValue;";
+  const rendered = renderSyntaxHighlightedDiff(diff, undefined, testTheme(), 2);
+  assert.match(rendered, /\x1b\[48;2;216;182;182m\x1b\[1mold/);
+  assert.match(rendered, /\x1b\[48;2;194;209;194m\x1b\[1mnew/);
+  assert.doesNotMatch(rendered, /\x1b\[48;2;(?:148;62;70|64;132;82)m/);
+});
+
 test("word emphasis marks low-overlap one-to-one changed pairs instead of skipping them", () => {
   const diff = "-1 out.push(pair.removed, pair.added);\n+1 block.push(next);";
   const rendered = renderSyntaxHighlightedDiff(diff, undefined, testTheme(), 2).split("\n");

src/diff/word/line-emphasis.ts

@@ -1,9 +1,13 @@
+import { bundledThemesInfo } from "shiki";
+import { codePreviewSettings } from "../../settings/index";
 import type { DiffWordEmphasis } from "../../settings/types";
 import { injectVisibleRanges } from "../../shared/terminal-text";
 import type { ParsedDiffLine } from "../parse";
 import { analyzeChangedLineBlock } from "./change-block";
 import { shouldEmphasizeChangedPair } from "./emphasis";
 
+const shikiThemeTypes = new Map(bundledThemesInfo.map((theme) => [theme.id, theme.type]));
+
 export function changedLineEmphasis(
   block: ParsedDiffLine[],
   wordEmphasis: DiffWordEmphasis,
@@ -50,5 +54,9 @@ function findCodeStart(line: string): number {
 }
 
 function wordEmphasis(kind: "add" | "remove"): string {
+  if (shikiThemeTypes.get(codePreviewSettings.shikiTheme) === "light") {
+    const bg = kind === "add" ? "\x1b[48;2;194;209;194m" : "\x1b[48;2;216;182;182m";
+    return `${bg}\x1b[1m`;
+  }
   return kind === "add" ? "\x1b[48;2;64;132;82m\x1b[1m" : "\x1b[48;2;148;62;70m\x1b[1m";
 }