I've installed a few free opencart themes for testing before I pay for any themes. Only 1 worked successfully, ie, when I enable it, my website changes.
However, none of the 3 themes I installed shows up in the admin area under Systems --> Settings --> General tab "Theme" drop menu. You can see the screenshot below:
According to some tutorials I saw, your installed themes are supposed to show up here. One tutorial I saw says its supposed to be under the 'store' tab. There are no 'Theme' drop menus there.
The only one that appears on my site is the Dark Velvet one. The Purple tree multivendor does not work, even when enabled.
Install methods I've used:
For Dark Velvet and Purple tree multivendor, I used the step by step instructions in the video here on the Purpletree's theme download site. Dark Velvet has basically the same instructions here but in text and less detailed.
Both themes downloaded as ocmod.zip files. In order to use the extensions --> installer in the OC admin, the theme needs to have ocmod.zip extensions. I saw other instructions say to extract the files to your computer then use Filezilla to upload it by clicking and dragging the files from your computer to the web host folder under catalog --> view --> themes (see here). I was able to do all the steps for a 3rd theme but it did not show up in OC admin or my site.
I got a free hosting account and subdomain for testing from 000webhost.com and used Filezilla as my FTP software and I use Windows 7.