docs: use david's most recent README from feature/vector_display branch

Replace lr_gradient_opposite() with msg_gradient() in render_message_overlay().
Messages now render in complementary colors matching the active theme:
- Green theme → magenta messages
- Orange theme → blue messages
- Purple theme → yellow messages

tests/test_config.py

@@ -164,6 +164,58 @@ class TestSetFontSelection:
         assert original_index == config.FONT_INDEX
 
 
+class TestActiveTheme:
+    """Tests for ACTIVE_THEME global and set_active_theme function."""
+
+    def test_active_theme_initially_none(self):
+        """ACTIVE_THEME should be None at module start."""
+        # Reset to None to test initial state
+        original = config.ACTIVE_THEME
+        config.ACTIVE_THEME = None
+        try:
+            assert config.ACTIVE_THEME is None
+        finally:
+            config.ACTIVE_THEME = original
+
+    def test_set_active_theme_green(self):
+        """Setting green theme works correctly."""
+        config.set_active_theme("green")
+        assert config.ACTIVE_THEME is not None
+        assert config.ACTIVE_THEME.name == "green"
+        assert len(config.ACTIVE_THEME.main_gradient) == 12
+        assert len(config.ACTIVE_THEME.message_gradient) == 12
+
+    def test_set_active_theme_default(self):
+        """Default theme is green when not specified."""
+        config.set_active_theme()
+        assert config.ACTIVE_THEME is not None
+        assert config.ACTIVE_THEME.name == "green"
+
+    def test_set_active_theme_invalid(self):
+        """Invalid theme_id raises KeyError."""
+        with pytest.raises(KeyError):
+            config.set_active_theme("nonexistent")
+
+    def test_set_active_theme_all_themes(self):
+        """Verify orange and purple themes work."""
+        # Test orange
+        config.set_active_theme("orange")
+        assert config.ACTIVE_THEME.name == "orange"
+
+        # Test purple
+        config.set_active_theme("purple")
+        assert config.ACTIVE_THEME.name == "purple"
+
+    def test_set_active_theme_idempotent(self):
+        """Calling set_active_theme multiple times works."""
+        config.set_active_theme("green")
+        first_theme = config.ACTIVE_THEME
+        config.set_active_theme("green")
+        second_theme = config.ACTIVE_THEME
+        assert first_theme.name == second_theme.name
+        assert first_theme.name == "green"
+
+
 class TestConfigDataclass:
     """Tests for Config dataclass."""