Update CSS

- all the white space should be click-able space and the minimum click-area per anchor should be around 44px x 44px on touch-devices. Thus the padding shall be added to each anchor instead of it's holder element
- why fix the position of the bar? it steals vertical screen-space which is valuable, especially on small screens -> absolute positioning instead of fixed
- bar overlays elements -> leave it small on big screens, push down the whole content on small screens and make it full-width
- icons have varying sizes on various pages -> set icon size to fixed pixel-dimensions
- toned down background color for the icons -> less obstrusive apperance