It's pretty simple posting videos from "youtube.com". With every video, they provide you with the code. You just copy the link code or embed code. With the Website Wizard, you will need to navigate to the page you want to edit, and then switch to the Code editor mode. Find your place in the code and insert the code from "youtube.com". Then publish your changes and view your site changes. The whole process takes about 1 to 2 minutes. The process should be similar with videos from other sites.
I'm not sure what you consider a "css box". You are using the Website Wizard, which is intended for beginners who don't know, or don't want to know, HTML/CSS. There is nothing wrong with using it, but you have to recognize the restrictions. In most cases, access to the style sheet in the Wizard would be counter productive.
While it is certainly possible to add in your own style settings, it's not very straight forward. What are you trying to do?
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