Extract .side-nav into a common file

framework/core/less/common/side-nav.less

@@ -0,0 +1,76 @@
+.side-nav > ul {
+  margin: 0;
+  padding: 0;
+  list-style: none;
+}
+
+// On phones, the index sidebar will pretty much take care of itself. The
+// navigation list is a .dropdown-select and will be shown as the title-
+// control; the new discussion button is the primary-control. On anything
+// larger than a phone, however, we need to affix the sidebar and expand the
+// .dropdown-select into a plain list.
+@media @tablet, @desktop, @desktop-hd {
+  .side-nav {
+    // Expand the dropdown-select component into a normal nav list.
+    & .dropdown-select {
+      display: block;
+
+      .expand-dropdown();
+
+      & .dropdown-menu {
+        .nav-list();
+      }
+    }
+  }
+}
+
+@media @tablet {
+  .side-nav {
+    padding: 15px 0;
+    white-space: nowrap;
+    overflow: auto;
+    -webkit-overflow-scrolling: touch;
+
+    &:after {
+      content: " ";
+      position: absolute;
+      left: 0;
+      right: 0;
+      margin-top: 15px;
+      border-bottom: 1px solid @fl-body-secondary-color;
+    }
+
+    & > ul > li, & .dropdown-menu > li {
+      display: inline-block;
+      margin-right: 25px;
+    }
+
+    & .dropdown-select .dropdown-menu > li > a .fa {
+      margin-right: 3px;
+    }
+
+    & .affix {
+      position: static;
+    }
+  }
+}
+
+@media @desktop, @desktop-hd {
+  .side-nav {
+    float: left;
+
+    &, & > ul {
+      width: 175px;
+    }
+    & > ul {
+      margin-top: 30px;
+
+      &.affix {
+        top: 56px;
+      }
+      & > li {
+        margin-bottom: 10px;
+      }
+    }
+  }
+}

framework/core/less/forum/app.less

@@ -21,6 +21,7 @@
 @import "@{common-path}/alerts.less";
 @import "@{common-path}/modals.less";
 @import "@{common-path}/layout.less";
+@import "@{common-path}/side-nav.less";
 
 @import "composer.less";
 @import "notifications.less";

framework/core/less/forum/index.less

@@ -1,81 +1,7 @@
 // ------------------------------------
 // Sidebar
 
-.side-nav > ul {
-  margin: 0;
-  padding: 0;
-  list-style: none;
-}
-
-// On phones, the index sidebar will pretty much take care of itself. The
-// navigation list is a .dropdown-select and will be shown as the title-
-// control; the new discussion button is the primary-control. On anything
-// larger than a phone, however, we need to affix the sidebar and expand the
-// .dropdown-select into a plain list.
-@media @tablet, @desktop, @desktop-hd {
-  .side-nav {
-    // Expand the dropdown-select component into a normal nav list.
-    & .dropdown-select {
-      display: block;
-
-      .expand-dropdown();
-
-      & .dropdown-menu {
-        .nav-list();
-      }
-    }
-  }
-}
-
-@media @tablet {
-  .side-nav {
-    padding: 15px 0;
-    white-space: nowrap;
-    overflow: auto;
-    -webkit-overflow-scrolling: touch;
-
-    &:after {
-      content: " ";
-      position: absolute;
-      left: 0;
-      right: 0;
-      margin-top: 15px;
-      border-bottom: 1px solid @fl-body-secondary-color;
-    }
-
-    & > ul > li, & .dropdown-menu > li {
-      display: inline-block;
-      margin-right: 25px;
-    }
-
-    & .dropdown-select .dropdown-menu > li > a .fa {
-      margin-right: 3px;
-    }
-
-    & .affix {
-      position: static;
-    }
-  }
-}
-
 @media @desktop, @desktop-hd {
-  .side-nav {
-    float: left;
-
-    &, & > ul {
-      width: 175px;
-    }
-    & > ul {
-      margin-top: 30px;
-
-      &.affix {
-        top: 56px;
-      }
-      & > li {
-        margin-bottom: 10px;
-      }
-    }
-  }
   .index-nav {
     & .new-discussion {
       display: block;