If you want to modify HotSpot Default theme go to Setup - Pages - Themes tab, Choose Default Theme.
Then you wil be able to load files that you want to use instead default logo, background image etc, which are placed at C:\Antamedia\HotSpot\www\Themes\Default folder.
If you want to create a completely different design of HotSpot pages, or to integrate it with existing website, you can modify pages directly.
This is done from Setup - Pages - Customize tab. Select Group of pages that you would like to edit and then from List select a page which you want to modify.
Page code will be loaded in the right side of window where you can do all required changes in code.
When you finish pres on Save button.
Press on Preview page in browser to see results of modifications in local browser.