We use the plugin Ninja Forms together with the Legenda Theme.
There we have the problem that if you use Safari the form isn’t usable because of missing checkboxes.
The Checkboxes are not showing up, but they are essential for the correct function of the form.
Under IE and Firefox the form ist running flawless.
We checked the problem with the support of ninja forms. They found out that if the default theme of wordpress is used the configuration is running well. So it seems to be a problem of the theme.
unfortunately our theme does not declare support of Ninja Forms plugin. If you want to use it with our theme – you need ask plugin developer for assistance or try to fix it on your own, if you have appropriate skills.