Theming – Why are all background colors of the theme disappearing when you switch to another server?

A site using MAYO 7.x-1.2 has been moved from a Windows IIS to a Linux server. Shortly thereafter, all the background colors defined in the MAYO settings (and also all custom CSS) disappeared. Also disappeared is the custom background color for dropdown menus – it just gets transparent. Apart from the background color, the main content and layout are fine. There may be no relation to this change on the server, but in no case could I debug this with anything I've tried. The PHP version is now 7.2, maybe that's related, and I'm not sure what it was under the windows server. The site must remain in D7 for the time being, although a D8 upgrade is planned.

Also note that I was aware that this is an old version of MAYO that would have needed to be updated long ago. I was hesitant with the update because I had changed the template file to get an extra scope and was not sure how to apply the fix while keeping the custom scope. (Could use some advice there.) When reading the security update documentation, the circumstances of the risk for this site do not seem to be accurate.

Advice on these topics is highly appreciated.