Multi-Domains and WP Multi-Network / separate themes directory


I use plugin Multi-Domains and WP Multi-Network. These plugins let me create other networks where I can use other domains. It works, but I have 1 problem:

I need to seperate themes directory. I'd like to have an other theme directory for every domain/network.

For now, every admin network has an access to all themes in themes directory.
I want to create networks for my clients, so there is no good situation.

Question: How in file .htaccess or wp-config add code like:

if {themes directory = "themes1"}
if {themes directory = "themes2"}