This tutorial explains the cause of and how to fix the error:
Watch the video explanation or see the written notes below for more:
About the missing style.css error:
- This error most often occurs when the wrong file has been chosen to upload.
- The download file is not the theme file. You must unzip the download file. You will find the theme .zip file inside.
- The download file will have a longer name, like "emilia-wordpress-theme.zip". The theme file (the one to upload) will have a short name, like "emilia.zip".
How to Unzip a file:
Video directions: https://www.youtube.com/watch?v=2L6qtSyW94E
Sometimes, especially on Windows PCs, you will be able to "look" inside the .zip folder when it has not be unzipped. Files inside a zipped file that has not been unzipped cannot be uploaded. Make sure you are picking a file from an unzipped file. Drag-and-drop the theme file onto your desktop to upload if you have to.
Learn more about how to unzip and install your theme: https://georgialoustudios.zendesk.com/hc/en-us/articles/360019515231-Install-WordPress-Theme