Hi there Oliver,
Thanks for getting in touch with us and sorry to hear about this issue. I’ll try to help you out on this, but please note that when it’s a theme issue the help we can provide is a bit limited.
First of all I think it will be best if you can reach out to the theme support folks and ask them if this is a known issue (maybe they even have a solution for it). While you contact them can you please send me the sire URL where I can see this? I might be able to extract some more info about the issue to add to the report you send them. Also, I couldn’t find any theme named ‘customis’ maybe you are referring to a custom theme you developed?
Please let me know about it,
Best,
Nico