Fix Catppuccin diff viewer contrast with colored text approach

Instead of using colored backgrounds which reduce contrast, now using:
- Near-black backgrounds (color232-233) for all diff types
- Colored text to differentiate between diff types:
  - Added lines: color151 (light green text) on color232
  - Removed lines: color210 (light red text) on color232
  - Changed lines: color189 (light text) on color232
  - Changed new: color210 (light red) on color232

This provides maximum contrast while still showing diff types clearly
through text colors rather than background colors. The dark backgrounds
ensure excellent readability without any toxic/vibrant colors.

Author: rafaelvzago

misc/skins/catppuccin256.ini

@@ -125,11 +125,11 @@
 
 [diffviewer]
     changedline = color231;color232
-    changednew = color231;color52
-    changed = color231;color233
-    added = color231;color22
-    removed = ;color88
-    error = color231;color160
+    changednew = color210;color232
+    changed = color189;color232
+    added = color151;color232
+    removed = color210;color232
+    error = color210;color233
 
 [widget-panel]
     sort-up-char = ↑