Cantrip's whole pitch is that you shouldn't have to think about design. Pick a theme, fill in your content, done. Most people who use it never open the "Custom CSS" page buried in a website's settings, and that's exactly how it's supposed to work.
But the field is there, and it surprises people when they stumble on it. It's not a color picker or a drag-and-drop style panel. It's a plain code editor: type raw CSS, hit save, and Cantrip drops it straight into the site's head tag on every page.
Who actually reaches for it. In practice: a freelance designer building a client's site who wants to override one button radius or spacing value the theme doesn't expose as a setting, or a business owner who picked up some CSS in a past job and wants a single tweak nobody else will ever notice. If neither is you, you'll probably never open this page, and there's nothing you're missing by skipping it.
Compare that to the alternatives. WordPress themes take custom CSS too, but doing it safely usually means setting up a child theme first, so a future update doesn't wipe your changes, then finding the right selector without breaking something else on the page. Squarespace lets you add CSS on its cheapest plan, but you need to step up a tier before you can inject any JavaScript alongside it. Wix keeps custom code behind a Premium plan, a connected domain, and turning off ads first. Cantrip skips the plan-tier hunt: the field is just there, on every website, because there's no page builder underneath fighting you for control of the markup.
That's also why it stays a plain text field instead of growing into a full visual style editor with sliders and swatches. The themes already handle color and spacing for the people who just want their site to look clean without learning anything. Building a second, half-finished visual tool on top of raw CSS would mean maintaining two ways to do the same job, and most builders that try end up with a worse version of both. If you want the real thing, you get the real thing.
One honest limit worth knowing: it applies to the whole website, not one page. Scoping a rule to just your contact page means writing a selector that targets it specifically, which works fine since Cantrip's markup is stable, but it's on you to write it. And since it saves as plain text with no linting, a typo won't break your site, but it also won't warn you either. Check the page after you save.
If you already know CSS, this is one less thing standing between what you want and getting it. If you don't, ignore the whole page. That's the product working the way it's supposed to, not something you're missing out on.