I'm getting an error "Can't upload CSS files" when I'm trying to create a theme.
I've followed these steps.
1) Downloaded the 2016 theme as "as_2015.zip" file
2) Extracted the .zip file, did few modifications in couple of CSS files from "core" folder
3) Created a .zip file of the "core" and "plugins" folders
4) Tried creating a new theme and selected the newly created .zip file, I'm getting the error "Can't upload CSS files"
5) Tried to select the downloaded "as_2015.zip" (with and without renaming) without any changes, getting the same error
What could be the issue?