Theming app does nothing

I have just upgraded to nextcloud 15.0.2 (from an older owncloud) and discovered the theming app. But this app does basically nothing: I can change logos and colors, but I cannot see any effect on the website.
Replacing the login logo gives me these messages:

{"reqId":"RvkSN3O4TS4vWoMJPHQU","level":0,"time":"2019-01-26T14:27:38+00:00","remoteAddr":"192.168.xx.yy","user":"xx.yy","app":"no app in context","method":"POST","url":"\/occal\/index.php\/apps\/theming\/ajax\/uploadImage","message":"No cache entry found for \/appdata_ochwcqlcljht\/theming\/images\/background.png (storage: local::\/owncloud\/data\/, internalPath: appdata_ochwcqlcljht\/theming\/images\/background.png)","userAgent":"Mozilla\/5.0 (Windows NT 6.1; Win64; x64; rv:64.0) Gecko\/20100101 Firefox\/64.0","version":"15.0.2.0"} {"reqId":"RvkSN3O4TS4vWoMJPHQU","level":0,"time":"2019-01-26T14:27:38+00:00","remoteAddr":"192.168.xx.yy","user":"xx.yy","app":"no app in context","method":"POST","url":"\/occal\/index.php\/apps\/theming\/ajax\/uploadImage","message":"No cache entry found for \/appdata_ochwcqlcljht\/preview\/2407 (storage: local::\/owncloud\/data\/, internalPath: appdata_ochwcqlcljht\/preview\/2407)","userAgent":"Mozilla\/5.0 (Windows NT 6.1; Win64; x64; rv:64.0) Gecko\/20100101 Firefox\/64.0","version":"15.0.2.0"} {"reqId":"RvkSN3O4TS4vWoMJPHQU","level":0,"time":"2019-01-26T14:27:38+00:00","remoteAddr":"192.168.xx.yy","user":"xx.yy","app":"no app in context","method":"POST","url":"\/occal\/index.php\/apps\/theming\/ajax\/uploadImage","message":"No cache entry found for \/appdata_ochwcqlcljht\/preview\/2407 (storage: local::\/owncloud\/data\/, internalPath: appdata_ochwcqlcljht\/preview\/2407)","userAgent":"Mozilla\/5.0 (Windows NT 6.1; Win64; x64; rv:64.0) Gecko\/20100101 Firefox\/64.0","version":"15.0.2.0"}

Browsing to the login screen gives me message like this:
{"reqId":"P795eWtsHfpMf9Dwe8hr","level":0,"time":"2019-01-26T14:33:01+00:00","remoteAddr":"192.168.xx.yy","user":"--","app":"core","method":"GET","url":"\/occal\/index.php\/login","message":"Scss is disabled for \/xxxxxx\/core\/css\/jquery-ui-fixes.scss, ignoring","userAgent":"Mozilla\/5.0 (Windows NT 6.1; Win64; x64; rv:64.0) Gecko\/20100101 Firefox\/64.0","version":"15.0.2.0"}
And I get the login page with the nextcloud logo and the default colors.

How can I make the theming app usable for me?

Mathias