Adding option for dark and light theme for the meilix ISO - (Solves -#431)

[pranav1698]

Checklist

• [x] I have read the Contribution & Best practices Guide and my PR follows them.
• [x] There is a corresponding issue for this pull request.
• [x] My branch is up-to-date with the Upstream master branch.
• [ ] The unit tests pass locally with my changes (provide a screenshot or link for test)
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)

Short description of what this resolves:

This adds an option to the user to choose between dark theme or light theme This resolves the frontend part,a pr for the backend part will be opened shortly

Changes proposed in this pull request:

• Adding a toggle for the light or dark theme

Fixes #431

Test deployment https://meilix-generator-432.herokuapp.com