diff --git a/__tests__/src/reducers/config.test.js b/__tests__/src/reducers/config.test.js
index c7c2a8345b..66271ebe73 100644
--- a/__tests__/src/reducers/config.test.js
+++ b/__tests__/src/reducers/config.test.js
@@ -1,6 +1,7 @@
 import { configReducer } from '../../../src/state/reducers/config';
 import ActionTypes from '../../../src/state/actions/action-types';
 import configFixture from '../../fixtures/config/export.example.json';
+import settings from '../../../src/config/settings';
 
 describe('config reducer', () => {
   describe('SET_CONFIG', () => {
@@ -55,6 +56,14 @@ describe('config reducer', () => {
     expect(configReducer({}, {
       state: { config: { new: 'stuff' } },
       type: ActionTypes.IMPORT_MIRADOR_STATE,
-    })).toEqual({ new: 'stuff' });
+    })).toMatchObject({ new: 'stuff' });
+  });
+  it('handles pre-existing functions in IMPORT_MIRADOR_STATE by deep merging those stems', () => {
+    const actual = configReducer(settings, {
+      state: { config: { new: 'stuff', theme: { palette: { mode: 'dark' } } } },
+      type: ActionTypes.IMPORT_MIRADOR_STATE,
+    });
+
+    expect(actual).toMatchObject({ theme: { ...settings.theme, palette: { ...settings.theme.palette, mode: 'dark' } } });
   });
 });
diff --git a/src/state/reducers/config.js b/src/state/reducers/config.js
index 090183b19f..d9ed49ad37 100644
--- a/src/state/reducers/config.js
+++ b/src/state/reducers/config.js
@@ -18,7 +18,13 @@ export const configReducer = (state = initialState, action) => {
     case ActionTypes.SET_CONFIG:
       return action.config;
     case ActionTypes.IMPORT_MIRADOR_STATE:
-      return action.state.config || {};
+      return {
+        ...(action.state.config || {}),
+        export: deepmerge(state.export || {}, action.state.config?.export || {}, { arrayMerge: overwriteMerge }),
+        requests: deepmerge(state.requests || {}, action.state.config?.requests || {}, { arrayMerge: overwriteMerge }),
+        theme: deepmerge(state.theme || {}, action.state.config?.theme || {}, { arrayMerge: overwriteMerge }),
+        themes: deepmerge(state.themes || {}, action.state.config?.themes || {}, { arrayMerge: overwriteMerge }),
+      };
     default:
       return state;
   }