docs: overhaul styling for example log messages on startpage

docs: overhaul styling for example log messages on startpage

 * Use brand-green for demo log messages on the startpage
   instead of the default `green` color defined by the browser vendor
   (after all, the brand green color is intended to reflect the
   boot status output)
 * Add a matching blue intro color (mixed using HSL from brand-green
   by 55deg hue rotation and 4% lightness)
 * Use a defined font-size
   (to avoid blurry rendering at 13.33333px, due to font-size: smaller)
 * Add possibility for the browser to show horizontal scrollbar
   (avoid overflow in mobile view)