Custom folder icons overwritten in bookmarks subfolders

### Describe the bug

Custom folder icon shows as it should in the bookmarks toolbar. However, the subfolders icons are replaced by the Firefox's defaults.

### Expected behavior

_No response_

### Screenshots

_No response_

### OS

Windows 10

### OS - Others

_No response_

### Firefox Version

143.0.3

### Distribution

- [x] [Original Lepton](https://github.com/black7375/Firefox-UI-Fix)
- [ ] [Lepton's photon style](https://github.com/black7375/Firefox-UI-Fix/tree/photon-style)
- [ ] [Lepton's proton style](https://github.com/black7375/Firefox-UI-Fix/tree/proton-style)

### Theme

- [ ] Light
- [ ] Dark
- [ ] Alpenglow (System: Light)
- [ ] Alpenglow (System: Dark)
- [ ] System Default
- [ ] Others: (e.g. [Photon Colors](https://addons.mozilla.org/en-US/firefox/addon/photon-colors/), [Google Chrome Dark](https://addons.mozilla.org/en-US/firefox/addon/google-chrome-dark/), [Fairytale Of Nature](https://addons.mozilla.org/en-US/firefox/addon/fairytale-of-nature/))

### Theme - More Info

_No response_

### `user.js` setup

user.js setup

```javascript
// Your settings
```

### Additional context

_No response_

Reproduce the issue in Firefox 143.0.3 on Windows 10 with the Original Lepton distribution, checking a custom bookmark folder and its subfolders. Trace the styling responsible for bookmark folder icons; done means custom icons remain visible in subfolders instead of being replaced by Firefox defaults.

Written by the indexing model from the issue text.