theme: Reduce icon grid spacing

Now that the icons are correctly scaled down and don't cause overlaps
with the spacing anymore, the spacing between icons causes them to be
smaller than they were before. This only affects low resolutions,
because the spacing property is used as a minimum spacing for the app
grid. On higher resolutions the spacing is always larger than this
minimum.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1375

data/theme/gnome-shell-sass/widgets/_app-grid.scss

@@ -7,7 +7,7 @@ $app_icon_padding: 24px;
 .icon-grid {
   -shell-grid-horizontal-item-size: $app_icon_size + $app_icon_padding * 2;
   -shell-grid-vertical-item-size: $app_icon_size + $app_icon_padding * 2;
-  spacing: $base_spacing * 6;
+  spacing: $base_spacing * 4;
 
   .overview-icon {
     icon-size: $app_icon_size;