Corners of cells don't render correctly

[kolibril13]

Describe the bug

When I run a sphinx-build with a minimal setup (only with extension myst_nb and all settings default),
the corners of cells don't render correctly.

zoom in:

next I added the sphinx copy button, but same problem:

next, I changed the theme to the "furo" theme, but same problem

Also with sphinx_book_theme

Reproduce the bug

I've made a repo that shows this problem:

https://kolibril13.github.io/manim_share/my_notebook.html

Here is the corresponding config file

https://github.com/kolibril13/manim_share/blob/main/conf.py

List your environment

myst-nb version v0.17.1

Also see Github pages workflow here:
https://github.com/kolibril13/manim_share/blob/main/.github/workflows/pages.yml