Add dark mode

README.md

app/assets/stylesheets/application.tailwind.css

app/controllers/application_controller.rb

app/helpers/application_helper.rb

app/javascript/application.js

app/models/user.rb

app/views/assignments/by_user.html.erb

app/views/assignments/index.html.erb

app/views/assignments/show.html.erb

app/views/conferences/index.html.erb

app/views/conferences/show.html.erb

app/views/conferences/stats.html.erb

app/views/devise/registrations/edit.html.erb

app/views/devise/registrations/new.html.erb

app/views/devise/sessions/new.html.erb

app/views/filedrop_files/download.html.erb

app/views/layouts/application.html.erb

app/views/sessions/_session.html.erb

app/views/sessions/show.html.erb

app/views/shared/_flash.html.erb