Feat: Overhaul in-game styles

[lijim]

This visual overhaul does several things:

• Reverted the change in #411 that caused each card in hand to have a fixed width:
• Added a background (credits SquareFrog on Pixabay: https://pixabay.com/vectors/cauldron-witches-potion-witchcraft-5524900/) to the in-game display to make the game feel less 'blank'
• Made cards have whiter / thin borders
• Made the chat box a little roomier
• Changed some colors for better contrast on a dark background
• Made the pass turn button more prominent and disabled styles for :active buttons when :disabled

Compare before:

to now: