Where can I find a theme kit to use for my websites, to skin them as html5 applications...
Sorry, I dunno how to explain myself.. I see this style in new web applications:
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Are you looking for styled GUI widgets? Try using the widgets in jQueryUI. You can use a theme from their Gallery, or create your own color scheme using their ThemeRoller. Either way, the library provides you with "modern" looking interface controls (tabs, sliders, date pickers, buttons, etc). There are also other widget frameworks out there, such as YUI from Yahoo.
Assuming that I am understanding the intent of your question, that's the best answer I can give other than hire a web designer. As Lese pointed out, this isn't really an HTML5 thing, and most sites are achieving it via hard working designers. These frameworks don't really provide templates that you can just apply to an existing page. They are somewhat skinnable building blocks that you can use as part of a larger site design.