Form input border referencing incorrect variable in theme-overrides

[jasonnrosa]

Description

Issue reported in dev Slack: https://hubspotdev.slack.com/archives/CMXUWUY5D/p1643168862060200

I found a small error in the boilerplate. On theme-overrides.css there is a variable {{ form_input_border }} under the form inputs styling, but it should be {{ form_field_border }} which is what's defined above.

Checklist

• [X] I have read the CONTRIBUTING document.
• [X] I have checked the issue tracker to make sure there isn’t already an open issue.