[website] CSS reset update

[fellyph]

Issue related: #3098

This pull request updates the CSS reset and improves the maintainability and consistency of the Playground website's styles. The main changes involve replacing the legacy CSS reset with a more modern and concise reset, centralizing color variables, and refactoring button styles to use CSS variables for easier theming.

CSS Reset and Base Styles:

• Replaced the old Meyerweb CSS reset with Josh Comeau’s custom CSS reset, resulting in a more modern, concise, and maintainable base style foundation. This includes new rules for box-sizing, margin resets, font smoothing, and improved text wrapping for headings and paragraphs.

CSS Variables and Theming:

• Centralized color and theme variables in the :root selector, adding new variables for white, shadow, and success colors, and moving WordPress admin color variables here for easier reuse.
• Updated the lazy load button styles (#lazy-load-initiator) to use the new CSS variables for color and shadow, improving consistency and making it easier to change the theme in the future.

Accessibility and Minor Fixes:

• Fixed a typo in the .sr-only accessibility class, correcting the syntax for the clip property.

Font and Layout Adjustments:

• Minor formatting adjustment to the font-family declaration in the body selector for improved readability.

Those changes should not affect the UI:

Before(trunk)

After