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docs/examples/styles/width.py

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+from textual.app import App
+from textual.widget import Widget
+
+
+class WidthApp(App):
+    CSS = """    
+    Widget {     
+        background: blue;
+        width: 50%;
+    }
+    """
+
+    def compose(self):
+        yield Widget()
+
+
+app = WidthApp()