test: Add test_theme_unregister

The commented out assertion may be added later after resolving #3619

tests/vegalite/v5/test_theme.py

@@ -46,6 +46,20 @@ def custom_theme() -> ThemeConfig:
     assert registered() == {"height": 400, "width": 700} == custom_theme()
 
 
+def test_theme_unregister() -> None:
+    @theme.register("big square", enable=True)
+    def custom_theme() -> ThemeConfig:
+        return {"height": 1000, "width": 1000}
+
+    assert theme.active == "big square"
+    fn = theme.unregister("big square")
+    assert fn is not None
+    assert fn() == custom_theme()
+    assert theme.active == theme.themes.active
+    # BUG: https://github.com/vega/altair/issues/3619
+    # assert theme.active != "big square"
+
+
 @pytest.mark.parametrize(
     ("color_code", "valid"),
     [