Added a class for RadioButtons container

with appropriate top padding...

IPython/html/static/notebook/js/widgets/widget_selection.js

@@ -126,8 +126,7 @@ define(["notebook/js/widgets/widget"], function(WidgetManager){
                 .hide();
             this.$container = $('<div />')
                 .appendTo(this.$el)
-                .addClass('widget-container')
-                .addClass('vbox');
+                .addClass('widget-radio-box');
             this.$el_to_style = this.$container; // Set default element to style
             this.update();
         },

IPython/html/static/notebook/less/widgets.less

@@ -178,6 +178,7 @@
 .widget-numeric-text {
     /* Single Line Textbox - used for IntTextView and FloatTextView */
     width : 150px;
+    margin : 0px !important;
 }
 
 .widget-progress {
@@ -261,6 +262,14 @@
     .align-start();
 }
 
+.widget-radio-box {
+    /* Contains RadioButtonsWidget */
+    .vbox();
+    .border-box-sizing();
+    
+    padding-top: 4px;
+}
+
 .docked-widget-modal {
     /* Horizontal Label */
     overflow: hidden;