Introducing columns formulas

tests/core/formatting/test_engine.py

@@ -243,7 +243,7 @@ class TestConflictResolution:
         css, formatted = engine.apply_format(rules, cell_value=150, row_data=row_data)
 
         assert isinstance(css, StyleContainer)
-        assert "var(--color-secondary)" in css.css  # Style from Rule 2
+        assert css.cls == "mf-formatting-secondary"  # Style from Rule 2
         assert formatted == "150.00 €"  # Formatter from Rule 1
 
         # Case 2: Condition not met (value <= budget)
@@ -282,7 +282,7 @@ class TestConflictResolution:
 
         css, formatted = engine.apply_format(rules, cell_value=-5.67)
 
-        assert "var(--color-error)" in css.css  # Rule 3 wins for style
+        assert css.cls == "mf-formatting-error"  # Rule 3 wins for style
         assert formatted == "-6 €"  # Rule 4 wins for formatter (precision=0)
 
 
@@ -316,7 +316,7 @@ class TestWithRowData:
         css, _ = engine.apply_format(rules, cell_value=42, row_data=row_data)
 
         assert isinstance(css, StyleContainer)
-        assert "background-color" in css.css
+        assert css.cls == "mf-formatting-error"
 
 
 class TestPresets:
@@ -327,7 +327,7 @@ class TestPresets:
 
         css, _ = engine.apply_format(rules, cell_value=42)
 
-        assert "var(--color-success)" in css.css
+        assert css.cls == "mf-formatting-success"
 
     def test_formatter_preset(self):
         """Formatter preset is resolved correctly."""

tests/core/formatting/test_style_resolver.py

@@ -16,9 +16,14 @@ class TestResolve:
         assert result["font-weight"] == "bold"
 
     def test_resolve_preset_with_override(self):
-        """Preset properties can be overridden by explicit values."""
-        resolver = StyleResolver()
-        # "success" preset has background and color defined
+        """Preset CSS properties can be overridden by explicit values."""
+        custom_presets = {
+            "success": {
+                "background-color": "var(--color-success)",
+                "color": "var(--color-success-content)",
+            }
+        }
+        resolver = StyleResolver(style_presets=custom_presets)
         style = Style(preset="success", color="black")
         result = resolver.resolve(style)
 
@@ -66,6 +71,16 @@ class TestResolve:
 
         assert result == {}
 
+    def test_i_can_resolve_class_only_preset(self):
+        """Default preset with __class__ only is included as-is in resolve result."""
+        resolver = StyleResolver()
+        style = Style(preset="success")
+        result = resolver.resolve(style)
+
+        assert result["__class__"] == "mf-formatting-success"
+        assert "background-color" not in result
+        assert "color" not in result
+
     def test_resolve_converts_property_names(self):
         """Python attribute names are converted to CSS property names."""
         resolver = StyleResolver()
@@ -151,11 +166,11 @@ class TestToStyleContainer:
             None,
             ["background-color: red", "color: white"]
         ),
-        # Class only via preset
+        # Class only via preset (default presets use __class__, no inline CSS)
         (
             Style(preset="success"),
-            None,  # Default presets don't have __class__
-            ["background-color: var(--color-success)", "color: var(--color-success-content)"]
+            "mf-formatting-success",
+            []
         ),
         # Empty style
         (
@@ -246,3 +261,12 @@ class TestToStyleContainer:
         assert isinstance(result, StyleContainer)
         assert result.cls is None
         assert result.css == ""
+
+    def test_i_can_resolve_default_preset_to_container(self):
+        """Default preset with __class__ only generates cls but no css."""
+        resolver = StyleResolver()
+        style = Style(preset="error")
+        result = resolver.to_style_container(style)
+
+        assert result.cls == "mf-formatting-error"
+        assert result.css == ""