perf(ui): improve web accessibility (#447)

- Make color and contrast meets WCAG 2 AA and above
- Fixes `aria-label` on button "copy link" in posts

_sass/addon/module.scss

@@ -107,7 +107,7 @@
 }
 
 %sidebar-links {
-  color: rgba(117, 117, 117, 0.9);
+  color: var(--sidebar-muted-color);
   -webkit-user-select: none;
   -moz-user-select: none;
   -ms-user-select: none;
@@ -122,6 +122,11 @@
   -webkit-box-orient: vertical;
 }
 
+%text-highlight {
+  color: inherit;
+  font-weight: 600;
+}
+
 /* ---------- scss mixin --------- */
 
 @mixin mt-mb($value) {