Set a bg color to the mainwindow

Might improve sharpness of font rendering
@see https://github.com/electron/electron/blob/master/docs/faq.md#the-font-looks-blurry-what-is-this-and-what-can-i-do

main.js

@@ -227,6 +227,7 @@ function createWindow() {
       minWidth: MIN_WIDTH,
       minHeight: MIN_HEIGHT,
       autoHideMenuBar: false,
+      backgroundColor: '#fff',
       webPreferences: {
         nodeIntegration: false,
         nodeIntegrationInWorker: false,