Well, after some research and time, I've came with the idea of installing a theme directly from "Get New Themes" on the configuration screen and then modifying them, so here are the instructions:
1.- Open a terminal and type:
2.- Introduce your user password when asked for it to
3.- Go to the tab "Theme" on the Login Screen" windows that opens
4.- Select "Get New Themes" and choose the one you like the most and install it
5.- After it's installed open Dolphin (Or your favorite File Manager) and navigate to
6.- Open the folder of the theme you installed
7.- In this folder you'll find the background image of the theme (Usually named "background") and any other images that the theme use, these are the files you must replace, so you can copy as root an image and rename it "background" or the file name of the image the theme has as background, but be careful that the image you copy has the same file extension as the original, for example: If the original background image is "background.png" you must copy a PNG image, else it won't work.
8.- Once you have modified all the images you want, select the theme on the Login Screen configuration window and hit Apply. Now, to check if it worked just close your current session so you can go to the Login Screen.
This worked for me, maybe it's not a solution for installing themes but it worked for what I wanted to do. I guess you can also modify the code of the xml file to fully modify the theme.
So, that's it I hope this can help someone.