Add default color for code blocks' span (prisma#7080)

Author: carlagn

src/theme/CodeBlock/Content/styles.module.scss

@@ -30,6 +30,7 @@
   min-width: 100%;
   > span {
     padding: 0 0 0 calc(var(--ifm-pre-padding) * 2);
+    color: var(--primary-font-color);
   }
 }