Dark mode

[Mozzo1000]

The web app is using a lot of bright colors (mostly white) and some people (myself included) prefer more dark mode in most situations. The user should have the option to toggle dark/light mode.

Fortunatly we are using tailwind for the frontend and that makes it ridiculously easy to add dark mode.

Pages that need to have dark mode added to it:
[ ] - Index (home) - /
[ ] - Login - /login
[ ] - Register - /register
[ ] - Verify registration - /verify
[ ] - Library - /library
[ ] - Profile- /profile
[ ] - Books detail pages - /books/*
_{These pages do have some components that are shared between them.}

There is also the search bar that is not it's own page but only a component that need to be stylized appropriately for this as well.