feat(dark-theme): add --border-color variable

Author: zhensherlock

src/style/dark/var.scss

@@ -13,4 +13,5 @@
   --popper-footer-bg-color: #1d1e1f;
   --popper-bg-color: #1d1e1f;
   --fill-color: #262727;
+  --border-color: #1d1e1f;
 }