Add example for corner colors.

docs/examples/styles/scrollbar_corner_color.css

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+Screen {
+    overflow: auto auto;
+    scrollbar-corner-color: white;
+}

docs/examples/styles/scrollbar_corner_color.py

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+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 ScrollbarCornerColorApp(App):
+    def compose(self):
+        yield Label(TEXT.replace("\n", " ") + "\n" + TEXT * 10)
+
+
+app = ScrollbarCornerColorApp(css_path="scrollbar_corner_color.css")