In this StackBlitz example, I have used the above mentioned CSS to change the background color of the popup button. Please make sure to set the encapsulation to ViewEncapsulation.None to set the style locally.
Additionally, I also noticed that you opened a QnA Forum post which is completely okay. But, in case you were unaware, I wanted to inform you that with the DevCraft Complete license, you can also open a Technical Support ticket that has a 24 hour turnaround time (faster than the turnaround time for QnA post) and the ticket is private.
I hope this information helps. Please let me know if I can further assist you.