I use this code to make the background blue:
<!DOCTYPE html>
<html>
<body style="background-color:blue;">
</body>
</html>
Which works good but how can I make the background have two different colors? 50% to be blue and the rest 50% to be yellow for example, split horizontally...
Use a css gradient: https://css-tricks.com/css3-gradients/
background: linear-gradient(blue, yellow);
Or if you want don't want the gradual fade you have to set the position:
background: linear-gradient(to right, blue 0%, blue 50%, yellow 50%, yellow 100%);