fix: improve theme toggle visibility in light mode

[DarshanKudrigi]

Title: fix: improve theme toggle visibility in light mode

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

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