Quick answer: NO.
Longer answer: MAYBE. You can not just Buy and Import and have the template load into CSB. CSB does not allow the user to edit the HTML . CSB stores all the design information in the tld and tlx files. Then converts that to HTML during publishing.
BUT the good news is that often the images, styles, etc offered in many template packages CAN be implemented in CSB as separate components or in similar ways.
Many of us have taken "professionally designed" templates and revised to fit CSB. Do you have a specific template in mind? Maybe we could tell you how to make it or something similar work for your site.
To take an HTML template that works in another program can take just a few or MANY steps to convert to a CSB version. The whole process is hard to define in general terms because each template requires its own adjustments and treatment.
The key is usually the background images. If you use a top border and publish with frames turned on, then you will need an image that is cut to fit your top border. If you do not publish with frames, you can either use separate images for the border areas, or one large image for the whole page then put an invisible table over it.... Anyway.... Start with changing the background colors and images...
CSB also offers STYLES under FORMATTING. This allows for Headers, and other text formatting to be saved and reused quickly. (Most people do not use this feature, eeven though it is a great time-saver!)
Trudy used to make lots of templates, some from her own images shown here. She said on the Globalscape forum:
If you want to make a web design just for that one file, you don't have to save it separately. If you want to be able to use it for other files, though, you need to save it separately as a .tld file. When you start the new file, just keep the home page without adding any more pages, edit all the page layouts you want to edit using the Format>edit page layouts, and save the file as a .tld rather than a .tlx .
If you look up "page layouts" and "web designs" in the CSB help file, you'll find lots of suggestions, but generally it's something you learn by playing around with it.