Border style (#2292)

* border styles

* docs for border styles

* fix tests

* tests

* tests and docs

* changelog

* implement auto

* style information fix

docs/examples/styles/border_title_colors.css

@@ -0,0 +1,16 @@
+Screen {
+    align: center middle;
+}
+
+Label {
+    padding: 4 8;
+    border: heavy red;
+
+    border-title-color: green;
+    border-title-background: white;
+    border-title-style: bold;
+
+    border-subtitle-color: magenta;
+    border-subtitle-background: yellow;
+    border-subtitle-style: italic;
+}

docs/examples/styles/border_title_colors.py

@@ -0,0 +1,19 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Label
+
+
+class BorderTitleApp(App):
+    CSS_PATH = "border_title_colors.css"
+
+    def compose(self) -> ComposeResult:
+        yield Label("Hello, World!")
+
+    def on_mount(self) -> None:
+        label = self.query_one(Label)
+        label.border_title = "Textual Rocks"
+        label.border_subtitle = "Textual Rocks"
+
+
+if __name__ == "__main__":
+    app = BorderTitleApp()
+    app.run()