make the window thumbnails bigger

Adjust the layout of the overview and window thumbnails to make them
bigger. Also, make the background shade darker to compensate for the
increased thumbnail density.

https://bugzilla.gnome.org/show_bug.cgi?id=689876

data/theme/gnome-shell.css

@@ -613,7 +613,7 @@ StScrollBar StButton#vhandle:active {
 /* Overview */
 
 #overview {
-    spacing: 40px;
+    spacing: 24px;
 }
 
 #overview-group {
@@ -706,8 +706,8 @@ StScrollBar StButton#vhandle:active {
 }
 
 .window-picker {
-    -horizontal-spacing: 40px;
-    -vertical-spacing: 40px;
+    -horizontal-spacing: 32px;
+    -vertical-spacing: 32px;
 }
 
 /* Dash */

js/ui/overview.js

@@ -30,9 +30,9 @@ const GLSL_DIM_EFFECT_CODE = '\
    vec2 dist = cogl_tex_coord_in[0].xy - vec2(0.5, 0.5); \
    float elipse_radius = 0.5; \
    /* from https://bugzilla.gnome.org/show_bug.cgi?id=669798: \
-      the alpha on the gradient goes from 165 at its darkest to 98 at its most transparent. */ \
-   float y = 165.0 / 255.0; \
-   float x = 98.0 / 255.0; \
+      the alpha on the gradient goes from 250 at its darkest to 180 at its most transparent. */ \
+   float y = 250.0 / 255.0; \
+   float x = 180.0 / 255.0; \
    /* interpolate darkening value, based on distance from screen center */ \
    float val = min(length(dist), elipse_radius); \
    float a = mix(x, y, val / elipse_radius); \