It has been over 90 days after my marriage-based green card interview, which had gone very smoothly. I have absolutely not gotten any response or letter from USCIS. Is that a negative sign? What are my further actions?
Continue waiting; schedule an infopass or hire an immigration attorney.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.
You can schedule an infopass appointment to inquire about the delay. It seems a bit long. Generally, if there is such a delay, there is an issue. If you don't feel comfortable going to the infopass by yourself, hire a lawyer who would inquire for you.
This answer is for informational purposes only and should not be construed as legal advice.
If everything went smoothly as you say, then I'll advise that you schedule an infopass apointment and find out what's going on with your case. You cando that on USCIS.gov. If you don't feel comfortable doing that, you may hire an attorney solely for the purpose of following up on your case.