border classvars (#3097)

* border classvars

* changelog

* copy

* remove whitespace

* copy

docs/examples/guide/widgets/hello06.css

@@ -0,0 +1,12 @@
+Screen {
+    align: center middle;
+}
+
+Hello {
+    width: 40;
+    height: 9;
+    padding: 1 2;
+    background: $panel;
+    border: $secondary tall;
+    content-align: center middle;
+}

docs/examples/guide/widgets/hello06.py

@@ -0,0 +1,47 @@
+from itertools import cycle
+
+from textual.app import App, ComposeResult
+from textual.widgets import Static
+
+hellos = cycle(
+    [
+        "Hola",
+        "Bonjour",
+        "Guten tag",
+        "Salve",
+        "Nǐn hǎo",
+        "Olá",
+        "Asalaam alaikum",
+        "Konnichiwa",
+        "Anyoung haseyo",
+        "Zdravstvuyte",
+        "Hello",
+    ]
+)
+
+
+class Hello(Static):
+    """Display a greeting."""
+
+    BORDER_TITLE = "Hello Widget"  # (1)!
+
+    def on_mount(self) -> None:
+        self.action_next_word()
+        self.border_subtitle = "Click for next hello"  # (2)!
+
+    def action_next_word(self) -> None:
+        """Get a new hello and update the content area."""
+        hello = next(hellos)
+        self.update(f"[@click='next_word']{hello}[/], [b]World[/b]!")
+
+
+class CustomApp(App):
+    CSS_PATH = "hello05.css"
+
+    def compose(self) -> ComposeResult:
+        yield Hello()
+
+
+if __name__ == "__main__":
+    app = CustomApp()
+    app.run()