Extend dark mode, return sticky headers for assignments

app/views/assignments/_listview_date.html.erb

app/views/assignments/by_user.html.erb

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

app/views/devise/mailer/confirmation_instructions.html.erb

app/views/devise/mailer/email_changed.html.erb

app/views/devise/mailer/password_change.html.erb

app/views/devise/mailer/reset_password_instructions.html.erb

app/views/devise/mailer/unlock_instructions.html.erb

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

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

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

app/views/devise/shared/_error_messages.html.erb

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

app/views/layouts/application.html.erb