GUI bugfix (histogram) + touchup

- Fix the Histogram background and grid lines
- Add 1 pixel of empty space between in the Histogram panel
- Add a light grey color border for Gtk::DrawingArea oject, for those
using the render_frame method
- In the Toolbox, make the nested MyExpander and Gtk::Frame lighter, and
adjust the Gtk::DrawingArea background color accordingly
- All added class to the MyCurve has been removed, so that the
Gtk::Drawing object's style is used
- The Retinex Settings' expander is now level 2 for GUI consistency

rtdata/themes/RawTherapee.css

@@ -97,6 +97,7 @@ GtkDrawingArea {
     padding: 0;
     margin: 0;
     background-color: #363636;
+    border: 1px solid #252525;
 }
 
 GtkDrawingArea:selected {
@@ -183,13 +184,61 @@ GtkMenu {
     border-radius: 4px;
 }
 
+#ExpanderBox GtkDrawingArea {
+    background-color: #363636;
+}
+
+#ExpanderBox GtkFrame {
+    background-color: #3B3B3B;
+    border: 1px solid #2B2B2B;
+    border-radius: 4px;
+}
+
+#ExpanderBox GtkFrame GtkDrawingArea {
+    background-color: #3B3B3B;
+}
+
+#ExpanderBox GtkFrame  GtkFrame {
+    background-color: #414141;
+    border: 1px solid #313131;
+    border-radius: 4px;
+}
+
+#ExpanderBox GtkFrame  GtkFrame GtkDrawingArea {
+    background-color: #414141;
+}
+
 /* Sub-tool (MyExpander) background */
 #ExpanderBox2 {
     background-color: #3B3B3B;
-    border: 1px solid #505050;
+    border: 1px solid #484848;
     border-radius: 4px;
 }
 
+#ExpanderBox2 GtkDrawingArea {
+    background-color: #3B3B3B;
+}
+
+#ExpanderBox2 GtkFrame {
+    background-color: #414141;
+    border: 1px solid #565656;
+    border-radius: 4px;
+}
+
+#ExpanderBox2 GtkFrame GtkDrawingArea {
+    background-color: #414141;
+}
+
+#ExpanderBox2 GtkFrame GtkFrame {
+    background-color: #474747;
+    border: 1px solid #5C5C5C;
+    border-radius: 4px;
+}
+
+#ExpanderBox2 GtkFrame GtkFrame GtkDrawingArea {
+    background-color: #474747;
+}
+
 #MyExpanderTitle {
     font-size: 120%;
 }