Refactor settings to use stack to allow dark theme. Move theme to app reducer to allow persistence.

redux/app/app.actions.js

@@ -27,3 +27,8 @@ export const documentUploadFailure = (error) => ({
 export const toggleDeleteAfterUpload = () => ({
   type: AppActionTypes.TOGGLE_DLETE_AFTER_UPLOAD,
 });
+
+export const setTheme = (theme) => ({
+  type: AppActionTypes.SET_THEME,
+  payload: theme,
+});