There must be something different in the CSS applied by browser if you don't get the same result between two browsers.
Can you try a new feature from bookdown 0.21
::: {.theorem}
We have $a$ and $b$ and $c$.
We also have $d$ and $e$ and $f$.
:::
This is the new way for those environment - it uses Pandoc's fenced div to build custom block and we added support for bookdown's special env: bookdown/NEWS.md at main · rstudio/bookdown · GitHub
I am curious if Pandoc processing will solve this issue . Can you try ? Thanks!
About that: FAQ: How to format your code