Use Tailwind color variables instead of OKLCH values

[lokomilo]

Summary

This pull request updates app.css to use Tailwind color variables instead of OKLCH values.

Benefits

Improve readability

It is easier for developers to understand what var(--color-red-500) is going to look like than it is oklch(0.637 0.237 25.331).

Allow for easier customization

For example, if a developer wants to update the shade of gray their application uses, they can now perform a find and replace for neutral to slate, stone, or a shade of their choice.

This could potentially allow future extraction of something like a gray-shade variable, allowing developers to update the shade of gray used throughout their application by changing a single value.

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If you are interested in these changes, I am happy to open a similar pull request in the Vue starter kit repository too.

[SkandarS0]

the starter kit uses shadcn/ui components, therefore shadcn/ui uses oklch by default

[lokomilo]

@SkandarS0 fair enough, you're correct. However, shadcn/ui's docs do explicitly mention Tailwind - https://ui.shadcn.com/colors.

I'll leave the PR open for the maintainers to close or approve as they see fit.