Small css fix to remove white square in bottom right corner between scrollbars (#3233)

# Description of Changes

Please provide a summary of the changes, including:

- What was changed
Small css fix to remove white square in bottom right corner between
scrollbars
- Why the change was made
It's looking better
- Any challenges encountered
Nothing

| screens | light theme | dark theme |
|---|---|---|
| before| ![ligtht theme
before](https://github.com/user-attachments/assets/9c6d0c03-2796-422c-a398-742b3adf6e32)
| ![dark theme
befoe](https://github.com/user-attachments/assets/b183f6d3-d4dc-4cbc-82cc-20384b87e489)
|
| after | ![ligtht theme
after](https://github.com/user-attachments/assets/4675cff4-dafa-4f62-9a7e-4495760ea681)
| ![dark theme
after](https://github.com/user-attachments/assets/faedc00c-eb12-4787-b4b6-fcfa598f9fac)
|


Closes #(issue_number)

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [x] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.