Fix truecolor dim style

CHANGELOG.md

@@ -8,14 +8,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
-### Added 
-
-- Added `compact` parameter to `MaskedInput` https://github.com/Textualize/textual/pull/5952
-
 ### Fixed
 
 - Fixed `query_one` and `query_exactly_one` not raising documented `WrongType` exception.
 - Fixed logging to a file on Windows https://github.com/Textualize/textual/issues/5941
+- Fixed eight bit colors crashing when applying dim style https://github.com/Textualize/textual/pull/5957
 
 ### Changed
 
@@ -25,6 +22,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Added `Markdown.append` https://github.com/Textualize/textual/pull/5950
 - Added `Widget.release_anchor` https://github.com/Textualize/textual/pull/5950
+- Added `compact` parameter to `MaskedInput` https://github.com/Textualize/textual/pull/5952
 
 ## [3.7.1] - 2025-07-09
 

src/textual/filter.py

@@ -241,12 +241,12 @@ def truecolor_style(self, style: Style, background: RichColor) -> Style:
         """
         terminal_theme = self._terminal_theme
         color = style.color
-        if color is not None and color.is_system_defined:
+        if color is not None and color.triplet is None:
             color = RichColor.from_rgb(
                 *color.get_truecolor(terminal_theme, foreground=True)
             )
         bgcolor = style.bgcolor
-        if bgcolor is not None and bgcolor.is_system_defined:
+        if bgcolor is not None and bgcolor.triplet is None:
             bgcolor = RichColor.from_rgb(
                 *bgcolor.get_truecolor(terminal_theme, foreground=False)
             )

tests/test_filters.py

@@ -0,0 +1,29 @@
+from rich.color import Color as RichColor
+from rich.color import ColorType
+from rich.segment import Segment
+from rich.style import Style
+from rich.terminal_theme import MONOKAI
+
+from textual.color import Color
+from textual.filter import ANSIToTruecolor
+
+
+def test_ansi_to_truecolor_8_bit_dim():
+    """Test that converting an 8-bit color with dim doesn't crash.
+
+    Regression test for https://github.com/Textualize/textual/issues/5946
+
+    """
+    # Given
+    ansi_filter = ANSIToTruecolor(MONOKAI)
+    test_color = RichColor("color(253)", ColorType.EIGHT_BIT, number=253)
+    test_style = Style(color=test_color, dim=True)
+    segments = [Segment("This should not crash", style=test_style)]
+    background_color = Color(0, 0, 0)
+
+    # When
+    # This line will crash if the bug is present
+    new_segments = ansi_filter.apply(segments, background_color)
+
+    # Then
+    assert new_segments is not None