Fix incorrect default border color for TextInput

[brandongregoryscott]

Overview

Resolves #1581

For some reason we were applying a border: 1px solid transparent style in the base Input theme object, and then overriding it individually in the default appearance object with a borderColor prop. I don't think this was intentional, so I've made the default border color gray400 and explicitly set the transparent border in the none appearance object, and added a few tests. Aside from this, I believe there's a deeper issue in ui-box where shorthand props + individual props can conflict depending on when they are added to the stylesheet/used, but that's an issue to tackle another day.

Screenshots (if applicable)

FileUploader story

TextInput story (no change)

Documentation

• [ ] Updated Typescript types and/or component PropTypes
• [ ] Added / modified component docs
• [x] Added / modified Storybook stories

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