Refactor theme selection and preloading to avoid FOUC on theme change

[lewisdoesstuff]

Hi!

I noticed that the demo page flickers unstyled content when changing the theme from the menu.

I've refactored the theme switching JS to preload all of the themes at page load instead of having the browser fetch them dynamically by switching the stylesheet ref.

Avoids the flash of unstyled HTML when clicking an option in the menu.

This may make first page load slightly slower, but shouldn't be an issue once the styles are cached.

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[Nainish-Rai]

@lewisdoesstuff Hey I am testing this and will merge if all works fine

[lewisdoesstuff]

Did this work okay @Nainish-Rai ?

[Nainish-Rai]

@lewisdoesstuff Yess, thanks for the PR