[fix](@svelteui/core): fix dark-theme class inconsistency

[longnguyen2004]

Description

Making currentTheme reactive, so the wrapper div class syncs up with the html class. Partially fixes #334

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[BeeMargarida]

Hi! Thank you for this contribution! I'll not immediately merge since I want to analyze the issue again, since I'm not 100% sure what the cause yet.

[BeeMargarida]

Sorry, been busy with work, will check this out this week!

[BeeMargarida]

I can't seem to replicate the issue that this PR seems to fix. Can you?

[longnguyen2004]

Here's a StackBlitz repro of the issue https://stackblitz.com/edit/svelteui-dark-mode-bug-repro
And here's a video describing the bug

2023-05-13.17-47-07.mp4

Basically, if the initial theme state is dark, then some components will be stuck in the dark state, because currentTheme is not reactive, so the dark-theme class is stuck on in the wrapper div.

There's also the issue of the background being initially white, before switching to dark, which is due to this not being SSR friendly. I didn't include a fix for that here, but I've outlined potential fixes for it in #334 (comment)

[BeeMargarida]

After merging this I'll make a new release, and fixing this for good will be the focus of the next one, I think

[longnguyen2004]

While you're at it, have a look at #373 and #370 too, those are trivial fixes

[BeeMargarida]

While you're at it, have a look at #373 and #370 too, those are trivial fixes

Yap, looking at them now. I'll see if I can fix them and make a release this afternoon