Inserting CSS code to customize GUI of Sharepoint 365

Hi everyone,
We start using Sharepoint 365 (online). We used to work with Sharepoint Server 2019 (on-premise) version.
Using classic interface, we tried to customize navigation of Sharepoint with CSS code but could not find Script Editor webpart in menu. Using custom script options were already enabled in Sharepoint Admin Center. We don't plan to use modern interface for now.
Does anyone know how to fix that issue or another method to do?
Thanks in advanced.