css: tweak color scheme.

Light mode:

* Change child post border from orange back to dark yellow (still darker
  than previous yellow).
* Make flagged borders brighter red.
* Make admins brighter red.
* Make parent, child, and pending post notice bars brighter.
* Change copyright tags from purple to magenta (very close to copyright
  tag color from before).
* Darken forum topic new/approved/rejected labels.

Dark mode:

* Make platinum users brighter grey.

app/logical/color_palette.rb

@@ -10,14 +10,16 @@ module ColorPalette
     azure: 242,
     blue: 257,
     purple: 282,
+    magenta: 307,
   }
 
   SATURATIONS = {
     grey:   [17.5]*10,
     red:    [100, 80, 100, 100, 100, 90, 100, 90, 75, 50],
-    yellow: [60, 60, 60, 100, 100, 100, 100, 100, 100, 90],
-    green:  [40, 40, 50, 90, 100, 100, 100, 100, 100, 100],
+    yellow: [80, 60, 60, 100, 100, 100, 100, 100, 100, 90],
+    green:  [60, 40, 50, 90, 100, 100, 100, 100, 100, 100],
     purple: [100, 70, 90, 100, 100, 100, 100, 100, 80, 80],
+    magenta: [100, 70, 90, 100, 100, 100, 100, 100, 80, 80],
   }
 
   LIGHTNESSES = [