Set the width and height of the HTML and body to 100% each
Set the background color of the divs to black and set the height and width of the divs
Append the divs to the page. (If the divs are all in a line, make them into a grid using inline-block)
When the mouse moves over one of the divs, make it change to a different color (doesn't have to be random.) To do this, you will need to use a "forEach" loop and an event listener.
Keep working on your project until you're happy with it. You could make the blocks change to random colors, add more or less divs to the page, allow the user to pick which color the divs change to, etc.