Add tests for styles that have "sub-styles" and !important

This is a series of tests for checking styles that have sub-styles, or
sub-parts, or whatever the correct name would be; the testing being that if
!important is applied to the whole, that it works.

Starting with #2420 it became apparent that this didn't work as intended,
and once that work started it became obvious that it affected more than just
border.

So these tests test all of the styles that can be specified as a single
whole, or as a set of parts (sides, directions, etc).

tests/test_style_importance.py

@@ -0,0 +1,102 @@
+from textual.app import App, ComposeResult
+from textual.color import Color
+from textual.containers import Container
+from textual.css.scalar import Scalar, ScalarOffset
+
+
+class StyleApp(App[None]):
+    CSS = """
+    Container {
+        border: round green !important;
+        outline: round green !important;
+        align: right bottom !important;
+        content-align: right bottom !important;
+        offset: 17 23 !important;
+        overflow: hidden hidden !important;
+        padding: 10 20 30 40 !important;
+        scrollbar-size: 23 42 !important;
+    }
+
+    Container.more-specific {
+        border: solid red;
+        outline: solid red;
+        align: center middle;
+        content-align: center middle;
+        offset: 0 0;
+        overflow: scroll scroll;
+        padding: 1 2 3 4;
+        scrollbar-size: 1 2;
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        yield Container(classes="more-specific")
+
+
+async def test_border_importance():
+    """Border without sides should support !important"""
+    async with StyleApp().run_test() as pilot:
+        border = pilot.app.query_one(Container).styles.border
+        desired = ("round", Color.parse("green"))
+        assert border.top == desired
+        assert border.left == desired
+        assert border.bottom == desired
+        assert border.right == desired
+
+
+async def test_outline_importance():
+    """Outline without sides should support !important"""
+    async with StyleApp().run_test() as pilot:
+        outline = pilot.app.query_one(Container).styles.outline
+        desired = ("round", Color.parse("green"))
+        assert outline.top == desired
+        assert outline.left == desired
+        assert outline.bottom == desired
+        assert outline.right == desired
+
+
+async def test_align_importance():
+    """Align without direction should support !important"""
+    async with StyleApp().run_test() as pilot:
+        assert pilot.app.query_one(Container).styles.align == ("right", "bottom")
+
+
+async def test_content_align_importance():
+    """Content align without direction should support !important"""
+    async with StyleApp().run_test() as pilot:
+        assert pilot.app.query_one(Container).styles.content_align == (
+            "right",
+            "bottom",
+        )
+
+
+async def test_offset_importance():
+    """Offset without direction should support !important"""
+    async with StyleApp().run_test() as pilot:
+        assert pilot.app.query_one(Container).styles.offset == ScalarOffset.from_offset(
+            (17, 23)
+        )
+
+
+async def test_overflow_importance():
+    """Overflow without direction should support !important"""
+    async with StyleApp().run_test() as pilot:
+        assert pilot.app.query_one(Container).styles.overflow_x == "hidden"
+        assert pilot.app.query_one(Container).styles.overflow_y == "hidden"
+
+
+async def test_padding_importance():
+    """Padding without sides should support !important"""
+    async with StyleApp().run_test() as pilot:
+        padding = pilot.app.query_one(Container).styles.padding
+        assert padding.top == 10
+        assert padding.left == 40
+        assert padding.bottom == 30
+        assert padding.right == 20
+
+
+async def test_scrollbar_size_importance():
+    """Scrollbar size without direction should support !important"""
+    async with StyleApp().run_test() as pilot:
+        assert pilot.app.query_one(Container).styles.scrollbar_size_horizontal == 23
+        assert pilot.app.query_one(Container).styles.scrollbar_size_vertical == 42