🐞 Changing "Use Theme Background" does not apply to gutter

### Description

When disabling the "Use Theme Background" option, the gutter still uses the theme background rather than the standard background.

### To Reproduce

1. Enable a theme and use the theme background.
2. Observe the colour of the theme background.
3. Disable the "Use Theme Background" option.
4. Observe the colour of the gutter

### Expected Behavior

The gutter should also use the standard background as opposed to the theme background.

### Version Information

CodeEdit: 0.0.3-alpha (36)
macOS: 13.5

### Screenshots

在 macOS 上重現 CodeEditSourceEditor 中的主題背景切換，接著追蹤 gutter 背景的處理方式，以及控制主題背景的設定。完成的標準是：停用 “Use Theme Background” 後，編輯器和 gutter 都使用標準背景；payload 未指定要執行的特定檔案或測試。

由索引模型根據 Issue 內容生成。