On Linux, pick the color of the slider's thumbpart from the GTK theme....

third_party/WebKit/WebCore/platform/chromium/ScrollbarThemeChromiumLinux.cpp

Index: third_party/WebKit/WebCore/platform/chromium/ScrollbarThemeChromiumLinux.cpp
===================================================================
--- third_party/WebKit/WebCore/platform/chromium/ScrollbarThemeChromiumLinux.cpp	(revision 51794)
+++ third_party/WebKit/WebCore/platform/chromium/ScrollbarThemeChromiumLinux.cpp	(working copy)
@@ -33,6 +33,8 @@
 
 #include "PlatformContextSkia.h"
 #include "PlatformMouseEvent.h"
+#include "RenderTheme.h"
+#include "RenderThemeChromiumLinux.h"
 #include "Scrollbar.h"
 #include "TransformationMatrix.h"
 
@@ -73,6 +75,70 @@
     drawVertLine(canvas, rect.x(), rect.y(), bottom, paint);
 }
 
+static SkColor saturateAndBrighten(SkScalar* hsv,
+                                   SkScalar saturateAmount,
+                                   SkScalar brightenAmount)
+{
+    SkScalar color[3];
+    color[0] = hsv[0];
+
+    color[1] = hsv[1] + saturateAmount;
+    if (color[1] > 1.0)
+        color[1] = 1.0;
+    else if (color[1] < 0.0)
+        color[1] = 0.0;
+
+    color[2] = hsv[2] + brightenAmount;
+    if (color[2] > 1.0)
+        color[2] = 1.0;
+    else if (color[2] < 0.0)
+        color[2] = 0.0;
+
+    return SkHSVToColor(color);
+}
+
+static SkColor outlineColor(SkScalar* hsv1, SkScalar* hsv2) {
+  // GTK Theme engines have way too much control over the layout of the
+  // scrollbar. We might be able to more closely approximate its look-and-feel,
+  // if we sent whole images instead of just colors from the browser to the
+  // renderer. But even then, some themes would just break.
+  //
+  // So, instead, we don't even try to 100% replicate the look of the native
+  // scrollbar. We render our own version, but we make sure to pick colors that
+  // blend in nicely with the system GTK theme. In most cases, we can just
+  // sample a couple of pixels from the system scrollbar and use those colors
+  // to draw our scrollbar.
+  //
+  // This works fine for the track color and the overall thumb color. But it
+  // fails spectacularly for the outline color used around the thumb piece.
+  // Not all themes have a clearly defined outline. For some of them it is
+  // partially transparent, and for others the thickness is very unpredictable.
+  //
+  // So, instead of trying to approximate the system theme, we instead try to
+  // compute a reasonable looking choice based on the known color of the track
+  // and the thumb piece. This is difficult when trying to deal both with high-
+  // and low-contrast themes, and both with positive and inverted themes.
+  //
+  // The following code has been tested to look OK with all of the default GTK
+  // themes.
+  SkScalar min_diff = (hsv1[1] + hsv2[1])*1.2;
+  if (min_diff < 0.2)
+      min_diff = 0.2;
+  else if (min_diff > 0.5)
+      min_diff = 0.5;
+
+  SkScalar diff = fabs(hsv1[2] - hsv2[2])/2;
+  if (diff < min_diff)
+      diff = min_diff;
+  else if (diff > 0.5)
+      diff = 0.5;
+
+  if (hsv1[2] + hsv2[2] > 1.0)
+      diff = -diff;
+
+  return saturateAndBrighten(hsv2, -0.2, diff);
+}
+
 IntRect ScrollbarThemeChromium::trackRect(Scrollbar* scrollbar, bool)
 {
     IntSize bs = buttonSize(scrollbar);
@@ -89,10 +155,16 @@
     SkIRect skrect;
 
     skrect.set(rect.x(), rect.y(), rect.x() + rect.width(), rect.y() + rect.height());
-    paint.setARGB(0xff, 0xe3, 0xdd, 0xd8);
+    SkScalar track_hsv[3];
+    SkColorToHSV(RenderThemeChromiumLinux::trackColor(), track_hsv);
+    paint.setColor(saturateAndBrighten(track_hsv, 0, 0));
     canvas->drawIRect(skrect, paint);
 
-    paint.setARGB(0xff, 0xc5, 0xba, 0xb0);
+    SkScalar thumb_hsv[3];
+    SkColorToHSV(RenderThemeChromiumLinux::thumbInactiveColor(),
+                 thumb_hsv);
+
+    paint.setColor(outlineColor(track_hsv, thumb_hsv));
     drawBox(canvas, rect, paint);
 }
 
@@ -109,11 +181,14 @@
     const bool vertical = scrollbar->orientation() == VerticalScrollbar;
     SkCanvas* const canvas = gc->platformContext()->canvas();
 
+    SkScalar thumb[3];
+    SkColorToHSV(hovered
+                 ? RenderThemeChromiumLinux::thumbActiveColor()
+                 : RenderThemeChromiumLinux::thumbInactiveColor(),
+                 thumb);
+
     SkPaint paint;
-    if (hovered)
-        paint.setARGB(0xff, 0xff, 0xff, 0xff);
-    else
-        paint.setARGB(0xff, 0xf4, 0xf2, 0xef);
+    paint.setColor(saturateAndBrighten(thumb, 0, 0.02));
 
     SkIRect skrect;
     if (vertical)
@@ -123,10 +198,7 @@
 
     canvas->drawIRect(skrect, paint);
 
-    if (hovered)
-        paint.setARGB(0xff, 0xf4, 0xf2, 0xef);
-    else
-        paint.setARGB(0xff, 0xea, 0xe5, 0xe0);
+    paint.setColor(saturateAndBrighten(thumb, 0, -0.02));
 
     if (vertical)
         skrect.set(midx + 1, rect.y(), rect.x() + rect.width(), rect.y() + rect.height());
@@ -135,11 +207,12 @@
 
     canvas->drawIRect(skrect, paint);
 
-    paint.setARGB(0xff, 0x9d, 0x96, 0x8e);
+    SkScalar track[3];
+    SkColorToHSV(RenderThemeChromiumLinux::trackColor(), track);
+    paint.setColor(outlineColor(track, thumb));
     drawBox(canvas, rect, paint);
 
     if (rect.height() > 10 && rect.width() > 10) {
-        paint.setARGB(0xff, 0x9d, 0x96, 0x8e);
         const int grippyHalfWidth = 2;
         const int interGrippyOffset = 3;
         if (vertical) {