Hi, I'm a 3D Builder engineer. In the most recent update of 3D Builder we added a light theme. 3D Builder used to only have a dark theme, but now 3D Builder reflects your systems default theme selection.
If you go to Settings -> Personalization -> Colors -> Choose your default app mode -> Dark you will see the old 3D Builder theme that you are used to and seem to prefer.
Let me know if you have any other questions.
Thanks for reaching out!