webp main image and text color accessibility improvements

assets/css/app.scss

@@ -121,7 +121,7 @@ a {
   display: flex;
   align-items: center;
   text-align: center;
-  color: $gray-500;
+  color: $gray-400;
   font-family: $font-family-monospace;
   font-size: smaller;
   text-transform: uppercase;
@@ -152,9 +152,8 @@ a {
 }
 .post-lede,
 .post-body {
-  color: $gray-200;
   h2, h3, h4, h5, h6 {
-    color: $gray-300;
+    color: $gray-200;
     margin-top: 2rem;
   }
 }