Themes: Adapt themes to the dot grid background

Update the themes to the recent changes made to the node editor's dot
grid background:

1. Make sure themes aren't using a "Grid Levels" value of greater than 3

2. A lot of themes weren't updated to the dot grid at all, yet, and
still used only 2 "Grid Levels" leading to the old dot grid being hardly
visible. Those are changed to 3 "Grid Levels", as well, which most
closely matches the visual density of the old line grid.

3. Additionally the "Grid" colour of the "Deep Grey" theme is darkened
a bit to account for the dot grid being colored differently than the old
line grid, which lead to the dot grid being invisible.

presets/interface_theme/Deep_Grey.xml

@@ -464,7 +464,7 @@
     </view_3d>
     <graph_editor>
       <ThemeGraphEditor
-        grid="#303030"
+        grid="#383838"
         frame_current="#4c9933"
         time_scrub_background="#333333e6"
         time_marker_line="#b8b8b880"
@@ -955,7 +955,7 @@
         matte_node="#0076b6"
         distor_node="#292929"
         noodle_curving="2"
-        grid_levels="2"
+        grid_levels="3"
         dash_alpha="0.5"
         input_node="#3471ae"
         output_node="#cc6666"

presets/interface_theme/Maya.xml

@@ -947,7 +947,7 @@
         matte_node="#975b5b"
         distor_node="#749797"
         noodle_curving="6"
-        grid_levels="2"
+        grid_levels="3"
         input_node="#e64555"
         output_node="#b33641"
         filter_node="#584d80"

presets/interface_theme/Minimal_Dark.xml

@@ -955,7 +955,7 @@
         matte_node="#404040"
         distor_node="#404040"
         noodle_curving="5"
-        grid_levels="7"
+        grid_levels="3"
         dash_alpha="0.5"
         input_node="#404040"
         output_node="#404040"

presets/interface_theme/Print_Friendly.xml

@@ -955,7 +955,7 @@
         matte_node="#977474"
         distor_node="#749797"
         noodle_curving="0"
-        grid_levels="2"
+        grid_levels="3"
         input_node="#ff6675"
         output_node="#ff6675"
         filter_node="#6c696f"

presets/interface_theme/White.xml

@@ -948,7 +948,7 @@
         matte_node="#975b5b"
         distor_node="#c5ffff"
         noodle_curving="3"
-        grid_levels="2"
+        grid_levels="3"
         input_node="#ff4e5d"
         output_node="#0081ff"
         filter_node="#b09bff"

presets/interface_theme/XSI.xml

@@ -948,7 +948,7 @@
         matte_node="#79f2a0"
         distor_node="#eef279"
         noodle_curving="0"
-        grid_levels="2"
+        grid_levels="3"
         input_node="#ee8513"
         output_node="#ff8b00"
         filter_node="#ff0000"