Caching color blending in Opacity, ignoring ANSI colors

tests/renderables/test_opacity.py

@@ -1,9 +1,14 @@
 import pytest
+from rich.text import Text
+
+from tests.utilities.render import render
+from textual.renderables.opacity import Opacity
 
 
 @pytest.mark.skip
 def test_simple_opacity():
-    pass
+    text = Text("Hello, world!", style="#ff0000 on #00ff00")
+    assert render(Opacity(text, value=.5)) == ""
 
 
 @pytest.mark.skip