creating a multisite subdirectory using a modified stylesheet

Hi, have been going round in circles for several hours trying to find the solution to my problems, but just tying myself in knots!!
I have 2 probs as follows:

I am trying to create a multisite subdirectory as a copy of my main site (to which i can make changes) using a modified stylesheet of picture perfect. I have created my sub directory and activated the picture perfect theme but it just installs the original version. How can i get my subdirectory site to use the modiified css stylesheet.

Also i want to create additional subdirectory sites using the same content as my main site but with different themes. How am i able to do this.

Would appreciate any help you could give. Thanks