fix: viewport background gradient — Overlay0/Mantle 3-color gradient (#188)

Change viewport background from 2-color Base→Crust gradient to
3-color Overlay0→Mantle→Overlay0 gradient for better visual depth.

CatppuccinMocha/CatppuccinMocha.cfg

@@ -27,13 +27,13 @@
           <FCUInt Name="colorError" Value="4086016255"/>
         </FCParamGroup>
         <FCParamGroup Name="View">
-          <FCUInt Name="BackgroundColor" Value="505294591"/>
+          <FCUInt Name="BackgroundColor" Value="404235775"/>
-          <FCUInt Name="BackgroundColor2" Value="286333951"/>
+          <FCUInt Name="BackgroundColor2" Value="1819313919"/>
-          <FCUInt Name="BackgroundColor3" Value="404235775"/>
+          <FCUInt Name="BackgroundColor3" Value="1819313919"/>
-          <FCUInt Name="BackgroundColor4" Value="825378047"/>
+          <FCUInt Name="BackgroundColor4" Value="404235775"/>
           <FCBool Name="Simple" Value="0"/>
           <FCBool Name="Gradient" Value="1"/>
-          <FCBool Name="UseBackgroundColorMid" Value="0"/>
+          <FCBool Name="UseBackgroundColorMid" Value="1"/>
           <FCUInt Name="HighlightColor" Value="3416717311"/>
           <FCUInt Name="SelectionColor" Value="3032415999"/>
           <FCUInt Name="PreselectColor" Value="2497893887"/>