Fullscreen mode doesn't work in Safari

[mathbunnyru]

Describe the bug

context
I open https://sphinx-book-theme.readthedocs.io/en/latest/ using Safari on MacOS and press fullscreen button.

expectation
I expected Safari go to fullscreen mode.

bug
But instead nothing happens.

[Error] TypeError: document.documentElement.requestFullscreen is not a function. (In 'document.documentElement.requestFullscreen()', 'document.documentElement.requestFullscreen' is undefined)
	(анонимная функция) (sphinx-book-theme.js:1:1707)
	onclick (latest:382)```
**problem**

This is a problem for people using Safari.


### Reproduce the bug

1. https://sphinx-book-theme.readthedocs.io/en/latest/
2. Open FullScreen

### List your environment

macOS 12.2 under M1
Safari 15.3

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