You can google this, but here is how to enable SSH access to your phone:

1. enable developer mode (Settings > Security > Developer mode > toggle Developer mode on)
2. You will now have terminal app on the launcher screen. Open it.
3. type "passwd" and give a new password for the "user" account. Remember this password!
4. Now you can login to the phone from WIFI network. Find out the phone IP in your network and the username is "user". Use WinSCP to copy files to user home folder or PuTTY to access terminal.

You need root access to copy these css-files to system folders. Log in to your phone as "user" and type "devel-su" (password is "rootme").

Be careful! You are now root and can easily brick the phone. Do only things you are absolutely sure about.