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.

stubs/app/client/ui/CustomThemes.ts

@@ -0,0 +1,12 @@
+export type ProductFlavor = string;
+
+// TODO: move CustomTheme type outside of stub code
+export interface CustomTheme {
+  bodyClassName?: string;
+  wideLogo?: boolean;
+};
+
+export function getTheme(flavor: string): CustomTheme {
+  return {
+  };
+}