What's a “css theme” error in WordPress?
Usually, this error means that you're uploading the wrong files, so you just need to find the right ones, and you'll be good to go! If you extract the contents of the ZIP file you downloaded, you should see a main folder and inside you'll find a few other files and folders. One of those files should be called "Read Me" or something similar. Reviewing this should clarify which file to upload.
If you got an error message that there was the style.css sheet was missing, the main style sheet may be named something else, and/or is located in a separate css folder. You may have to redirect your software to locate the correct style sheet from this subfolder.
Still not working? Feel free to contact us!