Seafile is not exactly known for being able to fully customize the design. It is very easy. I have made a small tutorial on how to regularly exchange the background image of Seafile login using Unsplash Source.
The instructions are in German, but the principle is super simple. In the login.html of seahub we simply replace the call of the background image with a link to Unsplash Source https://source.unsplash.com/random/2400x1600.
See for yourself how easy it is.