feat: migrate css to scss

[Revathyvenugopal162]

PR Description

1. Post CSS Hooks and Configuration Files:

• Added PostCSS hooks in the configuration to integrate SCSS files smoothly.
• Updated conf.py to link the newly created SCSS files instead of the CSS files, ensuring proper styling.

closes #640

Reason

Reason:

• PostCSS allows for better automation, such as autoprefixing and minification of CSS, ensuring styles are processed consistently.

2. Converting All CSS to SCSS Files:

• Converted CSS files to SCSS for better structure, organization, and maintainability.
• SCSS allows the use of variables, mixins, and nesting, which improves code readability and reusability.

3. Renaming All References to CSS Files:

• Renamed all references to CSS files in __init__.py to match the new SCSS structure.

4. Converting Pixels to REM:

• Converted all pixel-based measurements to REM units for better scalability and responsiveness.
• For example, 16px was converted to 1rem (assuming a 16px root font size).

Reason and benefits

Reason:

• REM units are relative to the root element’s font size, which allows for more flexible and responsive scaling across different devices and user settings.

Benefits:

• Ensures scalability and consistency throughout the design, especially in responsive layouts and improved accessibility.

Important

I didn’t convert the ast-variables.scss and pydata-theme-custom.scss files to SCSS format in this PR because they are large files that I would like to test separately. It's crucial to ensure no details are missed during the conversion, as these files contain most of the theme modifications. I will handle the conversion in a separate PR later.

[jorgepiloto]

Thanks for adding this, @Revathyvenugopal162. We should use SCSS variables but let's do that in a separate pull-request. Thanks!