Refine the base design system

Problem/Motivation

When creating an Artisan subtheme, the theme comes with a few Tailwind CSS 4 variables overwritten. I think its better to just load Tailwind with its default values and maybe, add a comment to the official documentation showing how to overwrite them..

On the other hand, the stylesheets that ships with Artisan starterkit (such as base.css, form.css) should be using the @apply directive instead of just writing hand-made CSS, as it more compliant with Tailwind's workflow.

Proposed resolution

• Do not override TailwindCSS 4 variables default value.
• Use tailwind directives instead of writing custom CSS.