Add display options and set defaults

I request another option besides matching the OS theme.

Add another option for default display. When a user visits the site for the first time dark mode will be:
- Light Mode
- Dark Mode
- Match OS

I also recommend that the default options upon module installation be to:
- Enable 'Save in Cookies'
- Light Mode

Light mode should be the default because I would generally want my website theme to be shown to users. However I like the option to have the dark mode switch option for those who prefer.