Manywidgets don't support dark mode

MyST published notebooks support dark mode, but manywidgets appear not to.

| Light | Dark |
|--------|--------|
| <img width="1514" height="524" alt="Image" src="https://github.com/user-attachments/assets/71aa4502-caa3-4127-bd2f-56ccf6848a29" /> | <img width="1516" height="538" alt="Image" src="https://github.com/user-attachments/assets/b50a263b-97c1-4701-83b6-0bdcbfac7383" /> | 