Improve CSS highlighting of selected entry

[kardwen]

I repeatedly had troubles to find the current active entry in the navigation tree view on the left. In the following example "Intel" is highlighted and I overlooked it frequently when scrolling down on my 15,6 Inch FHD+ IPS display.

Maybe adding the highlight color to the background would already help. Another idea would be to put the sidebar in its own scrollable container that automatically displays the according section in the viewport.