Add example for background transparency.

docs/examples/styles/align_all.css

@@ -50,4 +50,4 @@ Label {
     width: auto;
     height: 1;
     background: $accent;
-}
+}

docs/examples/styles/background_transparency.css

@@ -0,0 +1,49 @@
+#t10 {
+    background: red 10%;
+}
+
+#t20 {
+    background: red 20%;
+}
+
+#t30 {
+    background: red 30%;
+}
+
+#t40 {
+    background: red 40%;
+}
+
+#t50 {
+    background: red 50%;
+}
+
+#t60 {
+    background: red 60%;
+}
+
+#t70 {
+    background: red 70%;
+}
+
+#t80 {
+    background: red 80%;
+}
+
+#t90 {
+    background: red 90%;
+}
+
+#t100 {
+    background: red 100%;
+}
+
+Screen {
+    layout: horizontal;
+}
+
+Static {
+    height: 100%;
+    width: 1fr;
+    content-align: center middle;
+}

docs/examples/styles/background_transparency.py

@@ -0,0 +1,20 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Static
+
+
+class BackgroundTransparencyApp(App):
+    """Simple app to exemplify different transparency settings."""
+    def compose(self) -> ComposeResult:
+        yield Static("10%", id="t10")
+        yield Static("20%", id="t20")
+        yield Static("30%", id="t30")
+        yield Static("40%", id="t40")
+        yield Static("50%", id="t50")
+        yield Static("60%", id="t60")
+        yield Static("70%", id="t70")
+        yield Static("80%", id="t80")
+        yield Static("90%", id="t90")
+        yield Static("100%", id="t100")
+
+
+app = BackgroundTransparencyApp(css_path="background_transparency.css")