More form control css variables

[adam-cove]

Overview

We need more variables to control some form properties

Bug report (delete if not a bug report)

It'd be great to add a few more variables to control

• default form field width
• form field background color
• form field foreground color

[jlews]

Form input widths are already in a variables, here they are with their default values:

  --arm-input-width: 23.4375rem;
  --arm-input-width-large: 26.25rem;

I've also added the following variables and created a PR to solve the other issues: #238

Here's the new variables with their defaults:

  --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);

If I've missed anything, let me know and I'll re-open the issue