Support styles with default style done

TODO

@@ -1,4 +1,4 @@
-1. Support styles and colors
+1. [X] Support styles with default style
 2. Support setting both horizontal and vertical gridareas
 3. Support events
 4. Remove unwanted sample ts files

src/grid-area.tsx

@@ -63,15 +63,20 @@ class GridArea extends PureComponent<GridAreaProps, {}> {
 
   static requiresSVG = true;
 
+  static defaultStyle = {
+    fill: 'rgba(0,0,0,0.25)'
+  }
+
   render() {
     const {
       innerWidth: width,
       innerHeight: height,
       marginTop: top,
       marginLeft: left,
-      style
     } = this.props;
 
+    const style = {...GridArea.defaultStyle, ...this.props.style}; 
+
     const isVertical = (this.props.direction === Direction.VERTICAL);
     const tickXAttr = isVertical ? 'y' : 'x';
     const tickYAttr = isVertical ? 'x' : 'y';