If you wanted to find the theme for the current page, you could get the current page and then access its theme by the Theme property.
Since you want to know the theme applied in the whole site, you will need to get all pages and take their themes. For this purpose, you will need to create an instance of CmsManager class. Then, get all pages and get their themes:
|CmsManager manager = new CmsManager();
|IList pages = manager.GetPages();
|foreach (ICmsPage page in pages)
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