Steps

1- Go to Firefox
2- Search Mozilla Add-ons
3- Enter the page https://addons.mozilla.org/en/firefox/extensions/
4- Go to "Extensions"
5- Search "Dark Reader"
6- Download and activate it

Now all sites are in dark mode, including scratch

7- Open Scratch online and enjoy your dark mode

cant you use scratch addons?

Could be

we all knew that...