fix(a11y): increased content-meta text contrast (#1980)

Author: saberzero1

quartz/components/styles/contentMeta.scss

@@ -1,6 +1,6 @@
 .content-meta {
   margin-top: 0;
-  color: var(--gray);
+  color: var(--darkgray);
 
   &[show-comma="true"] {
     > *:not(:last-child) {