ui: SCSS - create global color settings

[catlabs]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

We should create global settings for colors that are added to primary, accent and warning so we can use those ones in different components if needed.
Here is for instance the extra button's colors:

$button-theme: (
  alert: (
    color: #d32f2f,
    contrast: $light-primary-text
  ),
  alt: (
    color: #323232,
    contrast: $light-primary-text
  ),
  neutral: (
    color: #bfbfbf,
    contrast: $light-primary-text
  ),
  success: (
    color: #4caf50,
    contrast: $light-primary-text
  )
);

Expected behavior

We should rename this SCSS map and add all the extra colors in it.

[catlabs]

We also need a strategy on how to customize colors on components. Following Material, they use color="primary" on buttons for instance. I think we should keep this for consistency. From there, we have 2 options:

• Read the color attribute and add the right classes to the component (Material way). Backward is that we might need to add extra code in our ts
• Read the color and add the color to subcomponents (ex: dropdown inside language selector). Backward is that we use attribute selector in SCSS