1. If the screen size is less than 400px, change the background
color to blue.
2. If the screen size is greater than 400 px but less than 600px,
change the background color to red.
3. If the screen size is greater than 600px but less than 800px,
change the background color to green.
4. If the screen size is greater than 800px, change the background
color to yellow.