I would agree with Gareth that it should go in CustomCSS - I'm not sure whether CSS added to the AdditionalHTML settings would get cached with the other CSS styles?
Wrapped in <style> tags it will work within the AdditionalHTML and there would apply across whatever themes you are using, while CustomCSS applies to the specific theme (and if you are using several would need to be copied across to each of them). But as I said, I would go with Gareth's view that CSS should ideally be in CustomCSS, where possible. (There's another recent thread about adding javascript through the AdditionalHTML too - and whether it should be allowed - Not sure on that one as I don't think there's anywhere else to put it, while for CSS there is an appropriate setting for it, as well as some themes having a similar customLESS setting)
Query - if you don't have access to the admin pages, eg for adding customcss (It's in each theme's settings pages), then how do you add the AdditionalHTML, which is also an admin setting?
Richard