[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.

Reproduce the issue in VS Code using the Dracula theme and a snippet with a $1 placeholder. Inspect the theme configuration for selection-related colors and verify the selected text is visibly distinct from surrounding text while editing the snippet.

Written by the indexing model from the issue text.