Try to improve toolbar separator coloration for themed browser windows.

chrome/browser/themes/theme_service.cc

Index: chrome/browser/themes/theme_service.cc
diff --git a/chrome/browser/themes/theme_service.cc b/chrome/browser/themes/theme_service.cc
index 9f5b82a5d0c094aaec0878b5f62ddfe2adbdfcb7..09fab6d80ea953035e015cb7fc046087cd108c19 100644
--- a/chrome/browser/themes/theme_service.cc
+++ b/chrome/browser/themes/theme_service.cc
@@ -477,16 +477,28 @@ SkColor ThemeService::GetDefaultColor(int id, bool incognito) const {
       if (UsingDefaultTheme())
         break;
       return GetColor(ThemeProperties::COLOR_BOOKMARK_TEXT, incognito);
+    case ThemeProperties::COLOR_DETACHED_BOOKMARK_BAR_SEPARATOR: {
+      // Same as the normal toolbar separator, but with increased luminance.
+      // Either double it or halve the distance to 1.0, whichever is less of a
+      // difference.
+      color_utils::HSL hsl;
+      color_utils::SkColorToHSL(
+          GetColor(ThemeProperties::COLOR_TOOLBAR_BOTTOM_SEPARATOR, incognito),
+          &hsl);
+      hsl.l = std::min((hsl.l + 1) / 2, hsl.l * 2);
+      return color_utils::HSLToSkColor(hsl, SK_AlphaOPAQUE);
+    }
     case ThemeProperties::COLOR_DETACHED_BOOKMARK_BAR_BACKGROUND:
       if (UsingDefaultTheme())
         break;
       return GetColor(ThemeProperties::COLOR_TOOLBAR, incognito);

[Evan Stade, 2017/01/24 00:46:08]

tangent: I noticed that on many themes this is better if it's
COLOR_NTP_BACKGROUND, but if we made that change the text might not be readable
on it (for example, toolbar is dark text on a light bg and ntp is light text on
a dark bg)

[Peter Kasting, 2017/01/24 21:43:33]

I agree; see https://bugs.chromium.org/p/chromium/issues/detail?id=618335 ,
especially comment 9 and comment 12.

-    case ThemeProperties::COLOR_DETACHED_BOOKMARK_BAR_SEPARATOR:
+    case ThemeProperties::COLOR_TOOLBAR_BOTTOM_SEPARATOR:
       if (UsingDefaultTheme())
         break;
-      // Use 50% of bookmark text color as separator color.
-      return SkColorSetA(
-          GetColor(ThemeProperties::COLOR_BOOKMARK_TEXT, incognito), 128);
+      // Use 12.5% of bookmark text color as separator color.
+      return color_utils::AlphaBlend(
+          GetColor(ThemeProperties::COLOR_BOOKMARK_TEXT, incognito),
+          GetColor(ThemeProperties::COLOR_TOOLBAR, incognito), 0x20);
     case ThemeProperties::COLOR_NTP_SECTION_HEADER_TEXT:
       return IncreaseLightness(GetColor(kNtpText, incognito), 0.30);
     case ThemeProperties::COLOR_NTP_SECTION_HEADER_TEXT_HOVER: