style: align New Task popup with application theme

[kRyxar87t]

Related Issue

Closes #1151

Summary

This PR updates the New Task and New Subject modals to match the application's existing black-and-white design system.

Changes Made

• Updated modal background to match the application theme.
• Replaced the dark navy color palette with the shared light theme.
• Updated headings and labels to use the application's text colors.
• Restyled input fields and select elements using the existing design system.
• Updated secondary button styling to match the application's shared button styles.
• Removed duplicate modal CSS rules to keep the stylesheet clean and maintainable.
• Preserved the existing layout, spacing, animations, and functionality.

Testing

• Verified the New Task modal opens correctly.
• Verified the New Subject modal styling is consistent.
• Confirmed all inputs, buttons, and dropdowns function normally.
• Confirmed no layout or functionality changes were introduced.

Screenshots

before:

After:

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)