Studio is the content management system, or CMS, in which you build your course. This back-end tool is only accessible to members of the course team. The live course which participants access is the learning management system, or LMS.
These two systems serve different purposes, and so have different code that they run on, here and there. One area in which they differ is in their style sheets.
As a result, occasionally a customization you've made in Studio won't look the same in the live version of your class. This is because more strings of CSS are being applied to the live version than to Studio. Those which aren't outrightly styled in Studio will take on whatever customizations you add, but may not in the LMS.
Because of this difference, it is always a good idea to check how things look in the live version of your course, and to not rely on Studio displays.
When this happens, it indicates that something in the LMS' CSS style sheet is overriding this customization.
This isn't to say you can't customize your pages, but that you should use the live version of your class to spot-check your work.
If you don't want participants to see a unit you're working on, you can click on the "Preview" button on the right side of the page, or you can mark a unit as hidden and publish it, so that you can view this content in the live version, without allowing participants to see it.