Closes #539: Add dark mode and modernise navbar [create website]

[manciniedoardo]

Thank you for your Pull Request! We have developed this task checklist from the Development Process Guide to help with the final steps of the process. Completing the below tasks helps to ensure our reviewers can maximize their time on your code as well as making sure the admiral codebase remains robust and consistent.

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[github-actions]

Package	Line Rate	Health
admiraldev	98%	✔
Summary	98% (1264 / 1290)	✔

[manciniedoardo]

@bundfussr ok to merge?

[github-actions]

The website/documentation is availables at https://pharmaverse.github.io/admiraldev/539_dark_mode_and_navbar_menu_height/dev.