Wrong property key to generate a new theme in the Themes Editor page

[joaopalmeiro]

Hi! 👋

One of the property keys to generate a new theme/Extension via the createTheme()function in the Themes Editor page is wrong, if I'm correct. Instead of dark it should be theme.

Let me know your thoughts and if I can open a PR to fix this key, please. Thanks!

[jaywcjlove]

I have fixed this error, thank you.