Dark Mode Switch Improvements

Hi there,

I've been working on a personal D9 site that uses your module for a fun front-end theming challenge. In addition to needing D9 compatibility patches, I kept making changes to it to fit my needs. As a result, this is the first module I'll have contributed code to and I think I'd like to continue maintaining it so it can get an official D9 release. I've attached the patch and listed the changes I made below.

Problem/Motivation

• Module does not have Drupal 9 compatibility
• Class is hard-coded and can't be updated
• Element that receives the class is hard-coded and can't be updated
• Storage of user preference relies on cookies, which can be problematic for sites needing to show a cookie acceptance dialog.
• Module does not account for system color mode settings
• jquery.once was a missing dependency for anonymous users

User interface changes

• Added an admin configuration page that allows the class name and the parent element to be set.

API changes

• Updated to be installed on Drupal 9.
• Explicitly includes jquery.once as a dependency.

Data model changes

• User preference relies on local storage instead
• On first visit, the user preference will first check if there's a system level color mode.