Dark mode, example environmental keys, updated contribution section

[lampealex888]

1. Dark Mode: Added support for dark mode, enhancing readability and reducing eye strain for users who prefer a darker interface.
2. Example Environmental Keys: Included example environmental keys to provide clear guidance for setting up environment variables, facilitating a smoother onboarding process for contributors.
3. Updated Contribution Section: Enhanced the contribution section to ensure accuracy and clarity, making it easier for new contributors to understand how to get past common hurdles in contribution.

[ncbarta]

• academics/scheduling: Separators between courses are invisible. Is this intentional?
• Darkmode icon conflicts with sidebar (see: career/study-plan/), also it won't click when there's overlap.

• courses/: Filter control buttons are unreadable