Hey there! I'm curious as to what you're looking to customize? Crowdstack plans offer varying degrees of access to things like Custom CSS and Custom pages, as well as custom HTML in the header, but we may be able to help you if we know a bit more about your design goals.
Just a tidbit---Crowdstack uses Foundation framework for its CSS. Most browsers include a way to "inspect" a page to see the classes that are being used, which helps if you're trying to focus in on a particular piece of text or item on the page. In Chrome, you can right click on any part of the page to open "inspect" as a windowpane.
Also, if you do have access to Custom CSS, there are lots of CSS snippets and tips here in the community (just type CSS into the search bean at the top of the page). There are community members in here who have done some cool things leveraging the CSS.