Fix Catppuccin diff viewer with proper format and high contrast

- Used the same format as modarin256 which is known to work
- color231 (bright white) for foreground text
- Very dark backgrounds: color22 (added), color232/233 (changed), color88 (removed)
- Empty foreground for removed lines (;color88) following modarin256 pattern
- This should resolve the low contrast issue definitively

Test with: ./src/mc --skin=catppuccin256
Then use Ctrl+X D to open diff viewer between two files.

Author: rafaelvzago

misc/skins/catppuccin256.ini

@@ -124,12 +124,12 @@
     viewselected = color228;color189
 
 [diffviewer]
-    added = color255;color233
-    changedline = color255;color232
-    changednew = color210;color232
-    changed = color255;color234
-    removed = color255;color235
-    error = color210;color232
+    changedline = color231;color232
+    changednew = color231;color52
+    changed = color231;color233
+    added = color231;color22
+    removed = ;color88
+    error = color231;color160
 
 [widget-panel]
     sort-up-char = ↑