You can find your default template’s CSS in
assets/templates/default/site.css 
otherwise (if you’ve created your own template), you’ll find it in
assets/templates/my_template_name/my_css_name.css
...or wherever you choose to place your css, it’s a question of method there.
Your MODx template is just like any web page using CSS for styling, you have a link to your stylesheet (the following is the default template’s code) :
<link rel="stylesheet" href="[(base_url)]assets/templates/default/site.css" type="text/css" media="screen" />
or it could be
<style type="text/css" media="screen">
@import url(path_to_my/stylesheet.css);
</style>
if you use the @import rule.
Your CSS is only stored in the database if you use the
CSS as document (also see
this) method, but that’s another method entirely, which I wouldn’t recommend for a newbie