move default (light) theme LESS source to a more appropriate place

add compact_dark theme variant

themes/compact_base.less

@@ -0,0 +1,31 @@
+/* rules specific to compact.css */
+
+body.ttrss_main.ttrss_index.flat {
+
+  #feedTree.dijitTree .dijitTreeLabel {
+    font-size : 13px ! important;
+  }
+
+  .dijitMenu .dijitMenuItemLabel,
+  .content-inner,
+  #content-insert,
+  .cdm .content,
+  .post .content {
+    font-size : 12px ! important;
+  }
+
+  div[id*=RROW] {
+    i.material-icons {
+      font-size: 18px;
+    }
+  }
+
+  .hl,
+  .post .header .title,
+  #floatingTitle a.title,
+  .cdm .title {
+    font-size : 13px ! important;
+  }
+
+
+}