templates/intro_ai.html: Customize "Intro to Artificial Intelligence"

The existing templates and styles weren't suitable for what I want, so
this commit needs to add custom CSS to `style.css`.
templates, style.css: Aesthetic changes, mainly concerning text size

I couldn't help seeing everything a bit too small. The previous default
text size of all the paragraphs was 8px and this commit changes it to 10px.