To get started, just link to the CSS file.
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/4.15/w3.css">
Other available files are...
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-2021.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-camo.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-flat.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-food.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-highway.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-ios.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-metro.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-safety.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-signal.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-vivid.css">
<link rel="stylesheet" type="text/css" href="https://assets.dah5.me.uk/w3css/w3-colors-win8.css">