Fix background color on navigation (#9010)

This PR fixes the following issue:
```
ich wollte nochmal melden, dass ich fast immer (wenn ich aus dem dashboard komme), einen weißen 3d-viewport und eine weiße pixel-leiste links habe
```


### URL of deployed dev instance (used for testing):
- https://___.webknossos.xyz

### Steps to test:
1. Switch on dark theme 
2. Open any dataset
3. Navigate back to dashboard through  "navbar"
4. Open any dataset
5. 3D Viewport should be black. Any 1px offset should also be
black/background color and not visible


### Issues:
- fixes https://scm.slack.com/archives/C5AKLAV0B/p1761125197020439

------
(Please delete unneeded items, merge only when none are left open)
- [ ] Added changelog entry (create a `$PR_NUMBER.md` file in
`unreleased_changes` or use `./tools/create-changelog-entry.py`)
- [ ] Added migration guide entry if applicable (edit the same file as
for the changelog)
- [ ] Updated [documentation](../blob/master/docs) if applicable
- [ ] Adapted [wk-libs python
client](https://github.com/scalableminds/webknossos-libs/tree/master/webknossos/webknossos/client)
if relevant API parts change
- [ ] Removed dev-only changes like prints and application.conf edits
- [ ] Considered [common edge
cases](../blob/master/.github/common_edge_cases.md)
- [ ] Needs datastore update after deployment

Co-authored-by: Norman Rzepka <[email protected]>

Author: hotzenklotz

frontend/javascripts/viewer/model/reducers/ui_reducer.ts

@@ -18,6 +18,7 @@ function UiReducer(state: WebknossosState, action: Action): WebknossosState {
         activeOrganization: state.activeOrganization,
         uiInformation: {
           ...defaultState.uiInformation,
+          theme: state.uiInformation.theme,
           storedLayouts: state.uiInformation.storedLayouts,
           navbarHeight: state.uiInformation.navbarHeight,
         },