Hello!
You should not modify files of the Clothing theme module.
Instead, you have the following options:
1. Using the Theme Tweaker module to add your styles. Here is
an example
2. Adding your styles through the
Custom Skin module
3. Creating your
custom theme based on Clothing and adding your styles there
You may also want to check this video:
http://kb.x-cart.com/display/XDD/Building+a+custom+skin+in+X-Cart+5