US24 // Styling overhaul

[matschi3]

• [x] use custom font (if possible a flexible/variable one)
• [x] setup correct font sizes on buttons, links, texts
• [x] choose 2-3 main colors with matching additional highlighting color (in addition to custom black/white)
• [x] find the correct place for new colors
• [x] define min/max sizes for Cards
• [x] style cards alá retro with black box-shadow (and gradient coloring)
• [x] add a main background to all pages (maybe animation if there is a matching one)
• [x] create mockups + clickthrue-.gif for Readme-file and update (Graduation Event)