You can use the <style>
tag and put your CSS inside. For example:
<html>
<head>
<style>
h1 {
color:red;
}
p {
color:blue;
}
</style>
</head>
<body>
...
</body>
</html>
Or you can write inline styles directly in your HTML elements, like:
<h1 style="color: red;">
Hello
</h1>