hackmd uses dark mode even when I configured it for using the system mode and my system is in light mode

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io

### Read our Official tutorial

It's on https://hackmd.io/c/tutorials/

或是中文版 https://hackmd.io/c/tutorials-tw/

### What's the problem

#### Current behaviour



Provide steps to reproduce:

1. Log in to hackmd on a light mode system
2. Change the settings to use the "system" style
3. hackmd is still in dark mode

#### Expected behaviour

hackmd should be in light mode in this configuration.

#### Environment

I use HackMD on:

**Desktop**
- OS: Linux
- Browser: Firefox
- Browser Version: 139.0.1

#### Additional context

Changing the settings to "light mode" works as expected.