tweak: improve mobile UX for sidebar links (#212)

Author: ycmjason

Diff for: lib/default-theme/SidebarLink.vue

@@ -69,6 +69,8 @@ a.sidebar-link
   border-left 0.25rem solid transparent
   padding 0.35rem 1rem 0.35rem 1.25rem
   line-height 1.4
+  width: 100%
+  box-sizing: border-box
   &:hover
     color $accentColor
   &.active