Fix RawScrollbar colors (#107177)

packages/flutter/lib/src/widgets/scrollbar.dart

@@ -1572,8 +1572,12 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     scrollbarPainter
       ..color = widget.thumbColor ?? const Color(0x66BCBCBC)
       ..trackRadius = widget.trackRadius
-      ..trackColor = _showTrack ? const Color(0x08000000) : const Color(0x00000000)
-      ..trackBorderColor = _showTrack ? const Color(0x1a000000) : const Color(0x00000000)
+      ..trackColor = _showTrack
+          ? widget.trackColor ?? const Color(0x08000000)
+          : const Color(0x00000000)
+      ..trackBorderColor = _showTrack
+          ? widget.trackBorderColor ?? const Color(0x1a000000)
+          : const Color(0x00000000)
       ..textDirection = Directionality.of(context)
       ..thickness = widget.thickness ?? _kScrollbarThickness
       ..radius = widget.radius

packages/flutter/test/widgets/scrollbar_test.dart

@@ -2465,6 +2465,52 @@ void main() {
     );
   });
 
+  testWidgets('RawScrollbar correctly assigns colors', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: RawScrollbar(
+              thumbVisibility: true,
+              thumbColor: const Color(0xFFF44336),
+              trackVisibility: true,
+              trackColor: const Color(0xFF2196F3),
+              trackBorderColor: const Color(0xFFFFEB3B),
+              controller: scrollController,
+              child: const SingleChildScrollView(
+                child: SizedBox(width: 4000.0, height: 4000.0),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0),
+          color: const Color(0xFF2196F3),
+        )
+        ..line(
+          p1: const Offset(794.0, 0.0),
+          p2: const Offset(794.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0xFFFFEB3B),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 90.0),
+          color: const Color(0xFFF44336),
+        ),
+    );
+  });
+
   testWidgets('trackRadius and radius properties of RawScrollbar can draw RoundedRectangularRect', (WidgetTester tester) async {
     final ScrollController scrollController = ScrollController();
     await tester.pumpWidget(