Added Theme Mode: Light Mode, Dark Mode and Auto Mode

[angelofan]

For #90 , #16 .

• Dark mode only detect dark class.
• Added automatic and manual switching of Theme Mode.
• Added pointer cursor for button in demo html.
• Added Dark Mode Compatibility in README.md.
• Added demo code link.

[angelofan]

This pull request is ready to review.

[hccullen]

Did you intend to close this? I wonder whether adding a parameter to the <pwa-install> would be better as it doesn't depend on some other element's class

[angelofan]

Did you intend to close this?

@hccullen After clarification from the maintainer, I tried to convince myself that this component should be consistent with the browser look and feel. So I closed this pull request.

I wonder whether adding a parameter to the <pwa-install> would be better as it doesn't depend on some other element's class

@hccullen Sure. My suggestion is to add an optional class parameter. That will only monitor the set class value after the given class value.

@khmyznikov Any insights?

[angelofan]

@hccullen Maybe I understand something different than what you described. You can open a new issue and describe it in detail.

[khmyznikov]

Same thoughts from me, I don't see why it should be implemented