freshen app/client/ui2018/cssVars.ts

The cssVars.ts file has changed to include some more knobs
for custom theming.  This commit updates the file, and
introduces a `stubs` directory for stubbing code that is
specific to our deployments of Grist and not of general interest.

app/client/tsconfig.json

@@ -1,6 +1,7 @@
 {
   "extends": "../../buildtools/tsconfig-base.json",
   "references": [
-    { "path": "../common" }
+    { "path": "../common" },
+    { "path": "../../stubs/app" },
   ]
 }