I received an offer letter but it was contingent on a drug test and a background check. I got a copy of the completed background check (eligible for hire) and I do not do drugs but I haven't heard anything since completing everything. I completed all of these things by December 11th and my start date is Jan 7th should I be worried that I haven't been contacted by anyone today is Jan 2nd.
Last half of December is usually when everyone takes their PTO so there is a good chance the HR person in charge of your paperwork just returned from their vacation. I would send a follow-up email to see what's up but chances are you will receive something in a day or two.
Try to call to HR. Happened something similar with me, and when I called HR they told me that the first "okay" that I gave was the most important. No needed a feedback about that for theirs and they're waiting for me in the day we have agreed