Update app background

[ZeroX-DG]

Description

This PR update the app background according to the theme so that when the sideBar is resized, the background color will be less noticeable.

Before	After

Issue fixed

#2790

Type of changes

• 🔘 Bug fix (Change that fixed an issue)
• ⚪ Breaking change (Change that can cause existing functionality to change)
• 🔘 Improvement (Change that improves the code. Maybe performance or development improvement)
• ⚪ Feature (Change that adds new functionality)
• ⚪ Documentation change (Change that modifies documentation. Maybe typo fixes)

Checklist:

• ⚪ My code follows the project code style
• ⚪ I have written test for my code and it has been tested
• ⚪ All existing tests have been passed
• 🔘 I have attached a screenshot/video to visualize my change if possible

[Rokt33r]

Awesome!