Very old Ecwid stores used the original look and feel which used images for the store buttons e.f. "Add to bag" etc.
Ecwid improved the look and feel of stores in 2015 and provided and option for the old stores to enable this called "Next-gen look and feel of product pages"
Users of old e-shop designer custom CSS themes should switch over to the new "next-gen" style now that the e-shop designer support is ending at the start of 2020. Doing this should avoid issues with the old style store buttons.
New options are available in the Ecwid Control Panel under "Settings" > "What's New":