Optional settings for font, layout, header, etc. missing in Settings

Trying to fix an older D7 site which uses the MAYO theme, and alternately, a custom version containing an extra region. The custom version worked as intended for years. Now, it works generally, but in theme Settings all the expandable options (Font, Layout, Style, Advanced Header Settings, Miscellaneous) do not display. They do display in the unmodified MAYO. What might cause these to disappear, when they previously were working?