Fix #1902: Set default electron color to black

If loading of the index.html is slow, there should be no white
background anymore.

electron/main.js

@@ -14,6 +14,7 @@ function createWindow(killall) {
   win = new BrowserWindow({
     show: false,
     backgroundThrottling: false,
+    backgroundColor: '#000000',
   });
 
   win.removeMenu();