refactor(Markdown): replace Sass with plain CSS and Tailwind @apply

Removes Sass dependencies from Markdown.scss and prism-theme.scss.
Converted to plain CSS files with @apply for Tailwind utilities.
Inlined all Sass variables, functions (getFontSize, getColor), and
color.adjust() calls as computed values. Replaced @include break
with plain @media (min-width: 768px). Added @reference directive
for Tailwind v4 @apply support.

Part of #8047.

Author: sujalgoel