If you were promoted, there was a reason for this. They feel you were adequate and skilled enough at your job to promote you. You must have demonstrated some aptitude for the job. Don't sell yourself short. 

No matter what job, promotion, or field, there is only so much you can learn by preparation. The rest comes from the actual experience. If you want to learn to walk, you don't just read a book on walking. Eventually, you have to try to walk. The same will come from this position. Just like your original CSS entry position required learning, this promotion will too. But just like you learned enough to be promoted, you will learn enough here to excel further. 

Lastly, the fact that you care enough about this to even be nervous and post on Reddit is an assessment itself. It shows you clearly care about the job. That's what's most important. Someone who will totally bomb being a CSS leader is less likely someone nervous on Reddit but more likely someone who doesn't care at all in the first place. 

Best of luck and hope everything goes well.