Render unchanged diff lines as grey instead of yellow

In Gitiles unified diff formatting (HtmlDiffFormatter), unchanged
context lines (starting with ' ') are wrapped in <span class="Diff-change">.
Previously, dark mode support introduced --diff-change styled as #960
in light mode and #fdd663 in dark mode, causing all unchanged lines
in unified diffs to render as yellowish-brown / yellow instead of neutral
grey.

Update --diff-change to #666 in light mode and #9aa0a6 in dark mode so
that unchanged context lines render in grey matching the secondary text
palette.

Add unit tests to verify that unchanged lines are formatted with
Diff-change and that --diff-change resolves to grey colors instead of
yellow in base.css.

Bug: b/562476907
google-bug-id: 562476907
TAG=agy
CONV=f9f02e8a-5730-4ba4-9c5c-f2415cbb4fd7

Change-Id: Ie3c687845b707161f462b5e5cfe10ce481e9fa2b

javatests/com/google/gitiles/DiffServletTest.java

diff --git a/javatests/com/google/gitiles/DiffServletTest.java b/javatests/com/google/gitiles/DiffServletTest.java
index 295418e..5d59a67 100644
--- a/javatests/com/google/gitiles/DiffServletTest.java
+++ b/javatests/com/google/gitiles/DiffServletTest.java
@@ -49,6 +49,8 @@
                 + " href=\"/b/repo/+/%s/foo\">b/foo</a>",
             c1.name(), c2.name());
     assertThat(actual).contains(diffHeader);
+    assertThat(actual).contains("<span class=\"Diff-change\"> foo</span>");
+    assertThat(actual).contains("<span class=\"Diff-insert\">+contents</span>");
   }
 
   @Test

javatests/com/google/gitiles/ThemeTest.java

diff --git a/javatests/com/google/gitiles/ThemeTest.java b/javatests/com/google/gitiles/ThemeTest.java
index 7e91a65..4b8cf5d 100644
--- a/javatests/com/google/gitiles/ThemeTest.java
+++ b/javatests/com/google/gitiles/ThemeTest.java
@@ -15,8 +15,10 @@
 package com.google.gitiles;
 
 import static com.google.common.truth.Truth.assertThat;
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static javax.servlet.http.HttpServletResponse.SC_NOT_FOUND;
 
+import com.google.common.io.Resources;
 import org.eclipse.jgit.internal.storage.dfs.DfsRepositoryDescription;
 import org.eclipse.jgit.internal.storage.dfs.InMemoryRepository;
 import org.eclipse.jgit.junit.TestRepository;
@@ -130,6 +132,19 @@
     assertThemeElementsPresent(html);
   }
 
+  @Test
+  public void diffChangeColorIsGreyNotYellow() throws Exception {
+    String css =
+        Resources.toString(
+            Resources.getResource(Renderer.class, "static/base.css"), UTF_8);
+    // Unchanged lines in unified diffs are formatted with class Diff-change.
+    // Ensure --diff-change resolves to grey in both light and dark modes instead of yellow.
+    assertThat(css).contains("--diff-change: #666;");
+    assertThat(css).contains("--diff-change: #9aa0a6;");
+    assertThat(css).doesNotContain("--diff-change: #960;");
+    assertThat(css).doesNotContain("--diff-change: #fdd663;");
+  }
+
   private static void assertThemeElementsPresent(String html) {
     assertThat(html).contains("id=\"gitiles-theme-toggle\"");
     assertThat(html).contains("Header-themeToggle");

resources/com/google/gitiles/static/base.css

diff --git a/resources/com/google/gitiles/static/base.css b/resources/com/google/gitiles/static/base.css
index d5054e7..8c21311 100644
--- a/resources/com/google/gitiles/static/base.css
+++ b/resources/com/google/gitiles/static/base.css
@@ -48,7 +48,7 @@
   --diff-hunk: #00c;
   --diff-delete: #c00;
   --diff-insert: #080;
-  --diff-change: #960;
+  --diff-change: #666;
   --diff-action-add: #060;
   --diff-action-delete: #600;
   --diff-action-rename: #006;
@@ -99,7 +99,7 @@
     --diff-hunk: #8ab4f8;
     --diff-delete: #f28b82;
     --diff-insert: #81c995;
-    --diff-change: #fdd663;
+    --diff-change: #9aa0a6;
     --diff-action-add: #81c995;
     --diff-action-delete: #f28b82;
     --diff-action-rename: #8ab4f8;
@@ -142,7 +142,7 @@
   --diff-hunk: #8ab4f8;
   --diff-delete: #f28b82;
   --diff-insert: #81c995;
-  --diff-change: #fdd663;
+  --diff-change: #9aa0a6;
   --diff-action-add: #81c995;
   --diff-action-delete: #f28b82;
   --diff-action-rename: #8ab4f8;