I’m actually using the Genesis eNews plugin on the Ambition Demo Homepage. Since you’re using the Ninja Forms for your newsletter, it loads the Plugin and all of it’s files/scripts on that page. The .one-third class is default CSS in the Genesis Framework, and it just so happens that Ninja Forms uses the same code in their plugin (which is why there is a conflict).
You could always add a special section in your CSS to handle customizations, or you could add it in your Genesis Themes Settings in “Header Scripts” –
<style>
@media only screen and (max-width: 800px) {
.five-sixths,
.four-sixths,
.one-fourth,
.one-half,
.one-sixth,
.one-third,
.three-fourths,
.three-sixths,
.two-fourths,
.two-sixths,
.two-thirds {
margin: 0 0 40px;
width: 100% !important;
}
}
</style>