Added separate border color to input

[dogusata]

by @awschristou: #340

Problem

• in themes where the default background color matches (or is similar to) the input box background, you cannot see the input box boundary
• when the input box has focus, its default color is a gradient scheme. UX direction wants this to be a standard border color

Solution

• allow hosts to define border colors for the input box, both for when the box has focus and when it doesn't:
  • mynah-color-text-input-border - the color of the input box border when the box does not have focus
  • mynah-color-text-input-border-focused - the color of the input box border when it does have focus
• these new styles have been given default values, so that:
  • the non-focused box appearance matches the current behavior (border matches background color)
  • the focused box appearance uses the default border color (no more gradient)

Screenshots

Before - left: unfocused input (can't see boundary), right: focused input (gradient)

After - left: unfocused input, right: focused input (both with sample colors)

Tests

• I have tested this change in the sample theme builder that is run with npm run dev

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.