fix dim

Author: willmcgugan

src/textual/filter.py

@@ -248,17 +248,18 @@ def truecolor_style(self, style: Style, background: RichColor) -> Style:
                     color.get_truecolor(terminal_theme, foreground=True)
                 )
                 changed = True
-            if style.dim:
-                color = dim_color(background, color)
-                style += NO_DIM
-                changed = True
 
         if (bgcolor := style.bgcolor) is not None and bgcolor.triplet is None:
             bgcolor = RichColor.from_triplet(
                 bgcolor.get_truecolor(terminal_theme, foreground=False)
             )
             changed = True
 
+        if style.dim and color is not None:
+            color = dim_color(background if bgcolor is None else bgcolor, color)
+            style += NO_DIM
+            changed = True
+
         return style + Style.from_color(color, bgcolor) if changed else style
 
     def apply(self, segments: list[Segment], background: Color) -> list[Segment]: