I think, with user interfaces, that clarity comes from using more than one distinction between similar items. This allows for various eyesight problems such as poor contrast, colour blindness and visual degradation (I myself have early Macular Degeneration) so using thickness, colour and dashed lines all gets my support. Looking messy doesn't bother me too much but looking cluttered does. I find the improvement made by WW in dark mode has significantly improved clarity for me but do accept others may find it less helpful.