improve theme toggle visibility in light mode

[DarshanKudrigi]

Description

Summary

Fixes the theme toggle visibility issue reported in [Issue #315](#315).

Problem

The theme toggle button was difficult to see in light mode because its white appearance blended into the light background.

Changes Made

• Improved the visibility and contrast of the theme toggle in light mode.
• Preserved the existing dark-mode appearance and theme-switching functionality.
• Kept the implementation minimal without changing the project structure or unrelated components.

Testing

• Verified toggle visibility in light mode.
• Verified toggle visibility in dark mode.
• Confirmed theme switching works correctly.
• Checked for unintended changes to other UI elements.
• Ran the relevant project checks.

Related Issue

Closes #315

Screenshots

Before

The theme toggle was difficult to see in light mode because the white button blended into the background.

After

The theme toggle is now clearly visible in light mode while preserving its appearance and functionality in dark mode.

The attached screenshots demonstrate the behaviour before and after the fix in both themes.

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