You can add or change the logo on top of the login screen as well as the footer and the message on the login screen under Configuration\Website Settings in your Lansweeper web console. Unfortunately, you will not be able to change the blue colour as it is actually a Windows login screen, using the Windows blue.
Changing the blue login screen has been requested by our customers before and is already on our customer wish list. I will tag this forum post as well to add weight to it. Do note that I cannot provide you with an estimated release date for when this might be possible within Lansweeper, unfortunately.