perf: improve <hr> visibility in dark mode (#1565)

Author: kungfux

_sass/addon/commons.scss

@@ -129,6 +129,11 @@ kbd {
   box-shadow: inset 0 -2px 0 var(--kbd-wrap-color);
 }
 
+hr {
+  border-color: var(--main-border-color);
+  opacity: 1;
+}
+
 footer {
   background-color: var(--main-bg);
   height: $footer-height;

_sass/colors/typography-dark.scss

@@ -104,10 +104,6 @@
     display: none;
   }
 
-  hr {
-    border-color: var(--main-border-color);
-  }
-
   /* categories */
   .categories.card,
   .list-group-item {