You can put your css in a separate text file. Then that file is typically put in a folder called css which is in the same folder where the HTML is. Then add a line to the <head> section of your HTML file to let it know where to find the css stlyes, like this:
<link rel="stylesheet" type="text/css" href="css/style.css">
which should work if your css is in a file called style.css
OR you could just put the styles in a <style> tag in the <head> tag of your HTML file, like
<style> .red { color: red; } </style>
It might be better to put all your styles for all your HTML pages into one css file because then you can reuse some of the styles on different pages.
RE: CSS for multiple HTML pages, one HTML page change