fix: ARM-201 - Additional variables added for form field styling

[jlews]

What's new?

Added the following variables for additional form styling flexibility:

  --arm-input-bg-color: var(--arm-color-white);
  --arm-checkbox-bg-color: var(--arm-input-bg-color);
  --arm-radio-bg-color: var(--arm-input-bg-color);

  --arm-input-fg-color: inherit;
  --arm-checkbox-fg-color: var(--arm-input-fg-color);
  --arm-radio-fg-color: var(--arm-input-fg-color);

Ticket number(s) in JIRA (if internal)

ARM-201

Checklist

• does this work have all the relevant tests?
• are your changes in Storybook?
• does everything have jsdoc?
• is everything exported from index.ts?