How to make a css file

Answer 1

First learn how to Code.
You need a text editor
Write code
And save it with extension
.css

Answer 2

Make a `foo.css` file and write in it your code like bellows:

body {
background-color: lightblue;
}

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Also you can put this code in your html file as:

<style>
body {
background-color: lightblue;
}
</style>

Answer 3

Save it with an extension .css