convert plain css to tailwindcss, refactoring

[dermatz]

• refactor existing css
• remove duplicate code
• convert plain css into TailwindCSS
• rearrange css
• unified CSS selectors
• improved main-menu hover-color for better accessibility
• unified main menu animations
• update dark-modes for gradient-box and tables
• unified line-heights and font-sizes

[DavidLambauer]

Thank you @dermatz !