Provide variables for theme-colors

Created by: kriim

Pull request #22836 introduced the two color maps colors and theme-colors:

$colors: (
  blue: $blue,
  indigo: $indigo,
  purple: $purple,
  pink: $pink,
  red: $red,
  orange: $orange,
  yellow: $yellow,
  green: $green,
  teal: $teal,
  cyan: $cyan,
  white: $white,
  gray: $gray-600,
  gray-dark: $gray-800
) !default;

$theme-colors: (
  primary: $blue,
  secondary: $gray-600,
  success: $green,
  info: $cyan,
  warning: $yellow,
  danger: $red,
  light: $gray-100,
  dark: $gray-800
) !default;

While updating a test project to v4-beta I noticed that it isn't possible anymore to override the theme-colors properly. To override theme-colors.primary one had to change the variable blue.

Would it be possible to introduce variables for the theme-colors which are then used in the color map? This way one could override single elements of the theme colors without doing strange things like $blue: red; or having to redefine the whole theme-colors map (while making sure that all other used variables are already defined, e.g. $green).

My suggestion is to do something like this:

$primary: $blue !default;
$secondary: $gray-600 !default,
$success: $green !default,
$info: $cyan !default,
$warning: $yellow !default,
$danger: $red !default,
$light: $gray-100 !default,
$dark: $gray-800 !default

$theme-colors: (
  primary: $primary,
  secondary: $secondary,
  success: $success,
  info: $info,
  warning: $warning,
  danger: $danger,
  light: $light,
  dark: $dark
) !default;