Replace deprecated functions

darken() and lighten() are global functions and as such are deprecated.
Replace them with `color.adjust()` which can change the lightness in the
same manner.

I'm following this PR.
jekyll/minima#849

Author: stevecheckoway

_sass/minima/_base.scss

@@ -1,5 +1,6 @@
 @charset "utf-8";
 
+@use "sass:color";
 @use "common" as *;
 
 /**
@@ -112,7 +113,7 @@ a {
   text-decoration: none;
 
   &:visited {
-    color: darken($brand-color, 15%);
+    color: color.adjust($brand-color, $lightness: -15%);
   }
 
   &:hover {
@@ -238,21 +239,21 @@ table {
   margin-bottom: $spacing-unit;
   width: 100%;
   text-align: $table-text-align;
-  color: lighten($text-color, 18%);
+  color: color.adjust($text-color, $lightness: 18%);
   border-collapse: collapse;
   border: 1px solid $grey-color-light;
   tr {
     &:nth-child(even) {
-      background-color: lighten($grey-color-light, 6%);
+      background-color: color.adjust($grey-color-light, $lightness: 6%);
     }
   }
   th, td {
     padding: calc($spacing-unit / 3) calc($spacing-unit / 2);
   }
   th {
-    background-color: lighten($grey-color-light, 3%);
-    border: 1px solid darken($grey-color-light, 4%);
-    border-bottom-color: darken($grey-color-light, 12%);
+    background-color: color.adjust($grey-color-light, $lightness: 3%);
+    border: 1px solid color.adjust($grey-color-light, $lightness: -4%);
+    border-bottom-color: color.adjust($grey-color-light, $lightness: -12%);
   }
   td {
     border: 1px solid $grey-color-light;

_sass/minima/_common.scss

@@ -1,5 +1,7 @@
 @charset "utf-8";
 
+@use "sass:color";
+
 // Define defaults for each variable.
 $base-font-family: -apple-system,
 BlinkMacSystemFont,
@@ -20,8 +22,8 @@ $text-color : #111 !default;
 $background-color: #fdfdfd !default;
 $brand-color : #2a7ae2 !default;
 $grey-color : #828282 !default;
-$grey-color-light: lighten($grey-color, 40%) !default;
-$grey-color-dark : darken($grey-color, 25%) !default;
+$grey-color-light: color.adjust($grey-color, $lightness: 40%) !default;
+$grey-color-dark : color.adjust($grey-color, $lightness: -25%) !default;
 $orange-color : #f66a0a !default;
 $table-text-align: left !default;
 // Width of the content area