[Bug]: user.css fetched from default user when Comfy started with --multi-user

### Prerequisites

- [x] I am running the latest version of ComfyUI
- [x] I have searched existing issues to make sure this isn't a duplicate
- [x] I have tested with all custom nodes disabled ([see how](https://docs.comfy.org/troubleshooting/custom-node-issues#step-1%3A-test-with-all-custom-nodes-disabled))

### What happened?

When activating --multi-user in comfy, the front end still tries to fetch `user.css` from default user.

It should instead probably fetch the actual user one.

### Steps to Reproduce

1. start comfy with `--mutli-user`
2. open comfy and setup your username
3. check network logs and see the 404 on user.css
4. add a `default` user with a 'user.css`
5. reload
6. notice the error is not there anymore

### How is this affecting you?

Minor inconvenience

### ComfyUI Frontend Version

1.24.4

### Browser

Chrome/Chromium

### Console Errors

```javascript
GET http://localhost:8190/api/userdata/user.css net::ERR_ABORTED 404 (Not Found)
```

### Logs

```shell

```

### Additional Context

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-4978-Bug-user-css-fetched-from-default-user-when-Comfy-started-with-multi-user-24f6d73d36508158a0aae989af5e1235) by [Unito](https://www.unito.io)