Blue glow around nodes is not colorblind friendly

[TSchwenn]

The pale blue glow around visited website nodes (introduced with toolness@aa0fe04) is difficult to see for this protanope colorblind user. The glowing visited and gray unvisited nodes are therefore difficult to discern. The glow color could simply be changed to a more saturated or darker blue, but ideally semantic coloring should always be reinforced by shapes or patterns. For example, make the visited nodes triangular, or make the borders of the unvisited nodes dashed.

[dethe]

Sabrina, if you have ideas about what we could do here, let me know.