Create a custom.css file. place your css code in there, and place link to that! below w3.css link, is the best option. Anything that exists in w3.ccs will be overridden by the same css in custom.css file, CSS = cascading style sheet.
If you two identical selectors
.selector { color: red;}
.selector { color: blue;}
The last one will always take precedence over previous. The same will happen if these were placed separately in css files, the selector in the css file link below previous will take precedence.