Fix Catppuccin diff viewer - maximum contrast with neutral backgrounds

The Catppuccin skin had insufficient contrast and backgrounds were still
too vibrant. Now using nearly identical grayscale backgrounds for maximum
readability:

- Added lines: color255;color233 (bright white on very dark gray)
- Changed lines: color255;color232 (bright white on near-black)
- Changed content: color255;color234 (bright white on dark gray)
- Removed lines: color255;color235 (bright white on slightly lighter gray)
- Errors: color210;color232 (pink accent on near-black)

This provides:
- Maximum contrast with bright white text
- Completely neutral backgrounds (no color vibrancy)
- Subtle differentiation through gray shades (232-235)
- Maintains Catppuccin identity through accent colors only

The diff viewer is now highly readable with no distracting colors.

Signed-off-by: Rafael Zago <rafaelvzago@gmail.com>

Author: rafaelvzago

misc/skins/catppuccin256.ini

@@ -124,11 +124,11 @@
     viewselected = color228;color189
 
 [diffviewer]
-    added = color255;color22
+    added = color255;color233
     changedline = color255;color232
     changednew = color210;color232
-    changed = color255;color17
-    removed = color255;color52
+    changed = color255;color234
+    removed = color255;color235
     error = color210;color232
 
 [widget-panel]