Can't change login background Nextcloud 12.0.5

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Nextcloud version 12.0.5
Operating system and version Ubuntu 16.04)
Apache or nginx version 2.4.25
PHP version 7.1

I’m new to Nextcloud. I installed Nextcloud with Snappy package. When i go to the Theme app, i’m not able to change the background. When i change it, it doesnt show any difference. The demo instead was working fine. Any suggestions for this issue?