Support dark mode

[Faryad8]

It would be very helpful if Intro.js could detect and adapt to dark mode automatically based on the host website's appearance.

Expected Behavior:

• When the host website is rendered in dark mode, the Intro.js tour should also appear in dark mode, without requiring any manual override.
• This detection should work regardless of how dark mode is enabled:
• CSS-based (e.g. using prefers-color-scheme)
• JavaScript-based toggles
• Browser extensions that enforce dark appearance

Requirements:

• The end user should not be able to toggle light/dark mode for the tour separately.
• Developers should be able to customize the dark mode theme, including:
• background color
• text color
• button styles
• highlight color
• (This flexibility is optional, but highly recommended.)

Notes:
The technical decision about whether to implement dark mode as part of the core or as a separate class system is left to the developers. The goal is seamless visual integration with the host site.