If you cannot install themes, update WordPress and similar on a Linux / *nix Localhost

WordPress cannot update or install on localhost unless the user rights are correct.

edit the file /etc/apache2/envvars

locate lines:

export APACHE_RUN_USER = www-data
export APACHE_RUN_GROUP= www-data

Change this to:

export APACHE_RUN_USER = YourUserName
export APACHE_RUN_GROUP= YourUserName

Then FTP works.

NB: the user must be one with the right to read, write and execute within the apache folders.

Thanx to this WordPress support page.