Add example for (background) scrollbar active/hover.

docs/examples/styles/scrollbars2.css

@@ -0,0 +1,8 @@
+Screen {
+    scrollbar-background: blue;
+    scrollbar-background-active: red;
+    scrollbar-background-hover: purple;
+    scrollbar-color: cyan;
+    scrollbar-color-active: yellow;
+    scrollbar-color-hover: pink;
+}

docs/examples/styles/scrollbars2.py

@@ -0,0 +1,19 @@
+from textual.app import App
+from textual.widgets import Label
+
+TEXT = """I must not fear.
+Fear is the mind-killer.
+Fear is the little-death that brings total obliteration.
+I will face my fear.
+I will permit it to pass over me and through me.
+And when it has gone past, I will turn the inner eye to see its path.
+Where the fear has gone there will be nothing. Only I will remain.
+"""
+
+
+class Scrollbar2App(App):
+    def compose(self):
+        yield Label(TEXT * 10)
+
+
+app = Scrollbar2App(css_path="scrollbars2.css")