when your phone is connected to your pc just make a new directory named "themes". (full path: /home/user/MyDocs/themes)

in this folder make two additional directories named "beta" and "beta-backup".

now place the two folder of the archive "images" and "backgrounds" in "beta".
disconnect usb.

open x-term.
1. get root:
type "sudo gainroot".
2. copy your files anywhere to back them up:
type "cp -r /usr/share/themes/beta/* /home/user/MyDocs/themes/beta-backup/"
3. overwrite existing files:
type "cp -r /home/user/MyDocs/themes/beta /usr/share/themes/"

(maybe you can use the .local-directory like when changing icons:
"you don't have to replace the files in /usr/share/icons/hicolor, you can use ~/.local/icons/hicolor instead (folder doesn't exist) to override default icons."
just adapt the paths?! haven't tried it yet.)