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..659adf52357a478eeb94cd1f59112e4ef2cade93 100644
--- a/chrome/browser/themes/theme_service.cc
+++ b/chrome/browser/themes/theme_service.cc
@@ -477,16 +477,27 @@ 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);
-    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 25% of bookmark text color as separator color.
+      return color_utils::BlendTowardOppositeLuma(

[Evan Stade, 2017/01/13 01:16:52]

The 50% didn't match up well with what the default theme did: 0xb6b4b6 for
normal, which is about 70% white (bg) and 25% black (text), and 0x282828 for
incognito, which is about 85% black (bg) and 15% white (text). So this now uses
75% bg and 25% text.

+          GetColor(ThemeProperties::COLOR_BOOKMARK_TEXT, incognito), 0xBF);
     case ThemeProperties::COLOR_NTP_SECTION_HEADER_TEXT:
       return IncreaseLightness(GetColor(kNtpText, incognito), 0.30);
     case ThemeProperties::COLOR_NTP_SECTION_HEADER_TEXT_HOVER: