If you are not satisfied with the default login themes,you can get a new one as you like by following step.
First,open Login Window manager at System–>Administration–>Login Window
Click on the Local tab, there are some themes in the selection box,you can also download some you satisfied from here.
Add the downloaded themes by clicking Add,then locate to the file and install, or just drag and drop the downloaded theme package into the themes selection box.
Select only or random login window theme at Theme drop-down menu,and tick on the themes you like.