how do i change the fullscreen fill colour?

edit

I have my program, in full screen, it has gray sidebars, its fine, but how can I change the fill colour from Gray, to black.

code that produced these results, is: If I have to edit the library for the background color, which opencv2 library is it? (I'll start digging around, but im not too hopeful to just stumble upon it.)

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <stdlib.h>

using namespace cv;
using namespace std;

Mat src; Mat src_gray;
int thresh = 50;
int max_thresh = 255;
RNG rng(12345);  //random number generator

/// Function header
void thresh_callback(int, void* );

/** @function main */
int main( int argc, char** argv ){

    VideoCapture cap;
    // open the default camera, use something different from 0 otherwise;
    if(!cap.open(0))
        return 0;
     // Create mat with alpha channel
    Mat mat(800, 800, CV_8UC4);     


for(;;){    
    Mat frame;
    cap >> frame;
    src = frame;
  /// Convert image to gray and blur it
  cvtColor( src, src_gray, CV_BGR2GRAY );
  blur( src_gray, src_gray, Size(3,3) );


  //createTrackbar( " Canny thresh:", "Source", &thresh, max_thresh, thresh_callback );
  thresh_callback( 0 , 0 );
  waitKey(200);
}
  return(0);
}

/** @function thresh_callback */
void thresh_callback(int, void* ){
  Mat canny_output;
  vector<vector<Point> > contours;
  vector<Vec4i> hierarchy;

  /// Detect edges using canny
  Canny( src_gray, canny_output, thresh, thresh*2, 3 );
  /// Find contours
  findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );

  /// Draw contours
  Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
  for(unsigned int i = 0; i< contours.size(); i++ ){
       Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
       drawContours( drawing, contours, i, color, 2, 8, hierarchy, 0, Point() );
     }

  /// Show in a window
  //namedWindow("Display frame", WINDOW_NORMAL);
  imshow( "Contours", drawing );

    cvNamedWindow("Name", CV_WINDOW_NORMAL);
    cvSetWindowProperty("Name", CV_WND_PROP_FULLSCREEN, CV_WINDOW_FULLSCREEN);
    imshow("Name", drawing);
}