I had the same problem installing new themes and found that uploaded files had been given the wrong UID/permissions.
So, as long as you have shell access, from the theme folder that isn’t working…
chown -R <your user name>.<your user group> .
chmod -R 644 *
chmod 755 .
chmod 755 <directories>
You need to do the last command for each directory.
Hope it helps!
