Using transient props in Flex component

[GomezIvann]

Checklist
(Check off all the items before submitting)

• Build process is done without errors and all tests pass in the /lib directory.
• Self-reviewed the code before submitting.
• Meets accessibility standards.
• Added/updated documentation to /website as needed.
• Added/updated tests as needed.

Description
To avoid passing some specific props that could be potentially HTML attributes, styled-components offer what they called transient props. These are regular props marked with the symbol $ so the library understands that they shouldn't be passed to the underlying component.

Screenshots

Additional context
What transient props are
https://medium.com/@probablyup/introducing-transient-props-f35fd5203e0c
https://dev.to/sarahscode/props-are-not-forever-preventing-props-from-being-passed-to-the-dom-with-styled-components-v5-1-l47

Closes #1343

[GomezIvann]

Also, I fixed a small error caught during validation in the Text Input styles, which were rendering false; when hovered: