Dark mode implementation

[Silverbullet069]

Resolve issue #18.

Light mode	Dark mode

My dark mode implementation is based on 3 techniques, as follows:

1. Using this CSS design pattern to add theme classes and dynamic variables inside main.css whose value is changed based on the changes of prefer-color-scheme: light|dark media query. These variables are referenced in other CSS files.
2. For SVG images that can't be dark-moded using CSS filter:, I've used this approach on W3School to create <picture> wraps around multiple <source media="(prefer-color-scheme: light|dark)" > in index.html.
3. Technique 1 and 2 are OS-level, they applied when system color scheme is changed via OS setting or browser dev tool. Based on someland2's answer on SOF, I've added a toggle button in the playground's header that when clicked can change theme at browser-level.

P/s: I've changed some nitty-gritty stuffs such as padding, margin, copyright year, id name, class name, ... hope it didn't cause much problem :")

P/s 2: There are 2 bugs, 1 at editor.js#L82 and 1 at modal.css#L65