v2 Sass (LMS): Set !default on all colors

Colors should be overridable.
See https://openedx.atlassian.net/browse/PLAT-1181

This sets all color variables to !default for v2 (Pattern Library)
Sass.

common/static/sass/edx-pattern-library-shims/base/_variables.scss

 // COLORS
-$light-gray1: rgb(221, 221, 221);
+$light-gray1: rgb(221, 221, 221) !default;
 
 
 // Font Sizes in em
@@ -127,15 +127,15 @@ $font-sizes: (
 
 // +Colors - UXPL new pattern library colors
 // ====================
-$uxpl-blue-base: rgba(0, 116, 180, 1); // wcag2a compliant
-$uxpl-blue-hover-active: darken($uxpl-blue-base, 7%); // wcag2a compliant
+$uxpl-blue-base: rgba(0, 116, 180, 1) !default; // wcag2a compliant
+$uxpl-blue-hover-active: darken($uxpl-blue-base, 7%) !default; // wcag2a compliant
 
-$uxpl-green-base: rgba(0, 129, 0, 1); // wcag2a compliant
-$uxpl-green-hover-active: lighten($uxpl-green-base, 7%); // wcag2a compliant
+$uxpl-green-base: rgba(0, 129, 0, 1) !default; // wcag2a compliant
+$uxpl-green-hover-active: lighten($uxpl-green-base, 7%) !default; // wcag2a compliant
 
-$uxpl-gray-dark: rgb(17, 17, 17);
-$uxpl-gray-base: rgb(65, 65, 65);
-$uxpl-gray-background: rgb(217, 217, 217);
+$uxpl-gray-dark: rgb(17, 17, 17) !default;
+$uxpl-gray-base: rgb(65, 65, 65) !default;
+$uxpl-gray-background: rgb(217, 217, 217) !default;
 
 
 // Alert styles
@@ -146,9 +146,9 @@ $warning-color-accent: rgb(255, 252, 221) !default;
 
 
 // CAPA correctness color to be consistent with Alert styles above
-$correct: $success-color;
-$partially-correct: $success-color;
-$incorrect: $error-color;
+$correct: $success-color !default;
+$partially-correct: $success-color !default;
+$incorrect: $error-color !default;
 
 
 // BUTTONS