Add dark mode and adapt colors to fit theme

[llanesluis]

Added next-themes package to provide dark-mode:

• 1st commit: Theme is based on user's preference
• 2nd commit: Theme selector component on footer matching style
• 3rd commit: Replaced hardcoded hex values for "accent" colors on tailwind.config.ts to be variables on globals.css, this way I added a different color for dark mode
• 4th commit: Change colors on several components to match theme automatically

[vercel]

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