Fix Bug 1454423 - update secondary text color to pass contrast test

* also add a tertiary text color for card context text
* secondary and tertiary text color are the same in default theme for now

system-addon/content-src/components/Card/_Card.scss

@@ -147,7 +147,7 @@
 
   .card-context {
     bottom: 0;
-    color: var(--newtab-text-secondary-color);
+    color: var(--newtab-text-tertiary-color);
     display: flex;
     font-size: 11px;
     left: 0;
@@ -157,7 +157,7 @@
   }
 
   .card-context-icon {
-    fill: var(--newtab-icon-secondary-color);
+    fill: var(--newtab-icon-tertiary-color);
     height: 22px;
     margin-inline-end: 6px;
   }

system-addon/content-src/styles/_theme.scss

@@ -26,6 +26,7 @@ body {
   --newtab-text-conditional-color: $grey-60;
   --newtab-text-primary-color: $grey-90;
   --newtab-text-secondary-color: $grey-50;
+  --newtab-text-tertiary-color: $grey-50;
   --newtab-textbox-background-color: $white;
   --newtab-textbox-border: $grey-90-20;
   @include textbox-focus($blue-60); // sass-lint:disable-line mixins-before-declarations
@@ -77,7 +78,8 @@ body {
   --newtab-link-secondary-color: $pocket-teal;
   --newtab-text-conditional-color: $grey-10;
   --newtab-text-primary-color: $grey-10;
-  --newtab-text-secondary-color: $grey-10-40;
+  --newtab-text-secondary-color: $grey-10-80;
+  --newtab-text-tertiary-color: $grey-10-40;
   --newtab-textbox-background-color: $grey-70;
   --newtab-textbox-border: $grey-10-20;
   @include textbox-focus($blue-40); // sass-lint:disable-line mixins-before-declarations