Just a note. Susan's suggestion and mine won't work very well together unless you have the snippet tag in one chunk, and separate chunks with nothing but the CSS code in them. The latter are the ones whose names are in the snippet. IOW, you don't want to send the snippet tag to regClientCSS(), just the CSS itself.
In fact, you might not need the first chunk, since you can put the snippet tag in the Template or the page content.
FYI, regClientCSS() will also take the URL of a CSS file rather than the raw CSS from a chunk as an argument if you prefer to use files for your CSS code.
$modx->regClientCSS(MODX_ASSETS_URL . 'css/mycssfile.css');