// After page loading, if the swap images are preloaded, mouse-over on a cell
// will immediately show the swap image, instead of momentarily handing for the swap image to be loaded.
// To test if preloading is working, delete the temporary cach file of the browser,
// quit the browser, re-open the browser, go to project page and move the mouse over a large cell (2x2)
// to see if there is any delay in loading the swap image.
// Colored rectangle background for each main menu
// Their positions, colors are defined in the style sheet.