Where can we specify custom CSS in Zoho Forms custom theme ?

Where can we specify custom CSS in Zoho Forms custom theme ?

      I'm using a form with a dark theme. The OTP popup window is unreadable, because for some reason, the OTP popup background fixes color to white, but still takes the font color specified in the custom theme. This ends up as white on white for me, rendering OTP nearly unuseable.

How can we either change the background color for the OTP popup, or specify CSS to address this.