[Bug/Enhancement] No highlight when selecting snippet placeholder

**Description**
When using the Dracula theme in VS Code, snippet placeholders (e.g. `$1`) can be filled normally. However, when selecting the inserted text with `Shift + ←/→`, the selected part has no visual distinction from the surrounding text. It looks like the selection is not applied.

https://github.com/user-attachments/assets/23ff244a-0c91-47c0-bcc0-804c4ab54f1c

https://github.com/user-attachments/assets/d7ded54e-fcff-4d85-bdeb-2958c51fd28f

**Steps to Reproduce**
1. Enable the Dracula theme in VS Code.
2. Insert a user snippet containing a `$1` placeholder.
3. Type into the `$1` position.
4. Use `Shift + ←/→` to select the text.
5. Observe the selection effect.

**Expected Behavior**
The selected text should be visually highlighted and clearly distinguished from the surrounding text.

**Actual Behavior**
The selected text has no visible highlight, making it indistinguishable from unselected text.

Draculaテーマと$1プレースホルダーを含むsnippetを使用して、VS Codeで問題を再現します。選択に関連する色についてテーマ設定を調べ、snippetの編集中に選択したテキストが周囲のテキストと視覚的に区別できることを確認します。

索引モデルが issue の本文から書いたものです。