Add a refreshing background to the app

Need a background which changes over time, ofcourse the background should be pokemon related :rofl:

[x] I ensure that this issue is not a duplicate of any current open issues(s).
[ ] Assign this issue to me (I want to work on this).
[ ] I will send my PR only after I'm assigned to this issue by any maintainer.
[ ] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

anshumanv / evolutions-webapp