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

redux/app/app.reducer.js

@@ -6,6 +6,7 @@ const INITIAL_STATE = {
   documentUploadInProgress: null,
   documentUploadError: null,
   deleteAfterUpload: false,
+  theme: "system",
 };
 
 const appReducer = (state = INITIAL_STATE, action) => {
@@ -43,6 +44,8 @@ const appReducer = (state = INITIAL_STATE, action) => {
         ...state,
         deleteAfterUpload: !state.deleteAfterUpload,
       };
+    case AppActionTypes.SET_THEME:
+      return { ...state, theme: action.payload };
     default:
       return state;
   }