Made the theme button toggable by keyboard
Added aria-label to many <a> without any text (usually, they contains Font Awesome icons)
Added aria-hidden whenever necessary
Labelled the "Copy URL and Description" contained in footer
Added an hidden label for the menu toggle for mobile accessibility
Changed some colors to meet the Color Contrast AA level: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
