Of course you need CSS, if user experience is succesfull in your site, maybe he will recomend, and why not, they can return many times, if you ignore the look of your site probably users dont return or you can create a bounce effect that can affect the SEO of your site. there are many things than affect SEO of your site, but more important is that your visitors have a good experience in every visit.
By the way, if you create a good experience in your site, you can obtain more conversions or client actions depending on your objetives, i think that is very difficult to gain without CSS.
Personally i check the w3schools contents for any question about web, i am a web designer, and every day i learn new things about this world.
