Add Styles.reset method revert internal attributes to None

src/textual/css/styles.py

@@ -235,6 +235,10 @@ class Styles:
         else:
             return None
 
+    def reset(self) -> None:
+        for rule_name in INTERNAL_RULE_NAMES:
+            setattr(self, rule_name, None)
+
     def extract_rules(
         self, specificity: Specificity3
     ) -> list[tuple[str, Specificity4, Any]]:

src/textual/css/stylesheet.py

@@ -114,7 +114,7 @@ class Stylesheet:
 
         _check_rule = self._check_rule
 
-        node.reset_styles()
+        node.styles.reset()
 
         # Get the default node CSS rules
         for key, default_specificity, value in node._default_rules:

tests/test_styles.py

@@ -0,0 +1,11 @@
+from rich.style import Style
+
+from textual.css.styles import Styles
+
+
+def test_styles_reset():
+    styles = Styles()
+    styles.text_style = "not bold"
+    assert styles.text_style == Style(bold=False)
+    styles.reset()
+    assert styles.text_style is Style.null()