Navigation on Browse Mode: "Grey out" non-link blocks

[scruffian]

In the Navigation section in Browse Mode, we should try showing non-link blocks in a greyed out state to make it clear that these don't represent site structure:

[MaggieCabrera]

The text colors in browse mode already look like the have contrast issues, how can we achieve this while keeping visibility? @jasmussen

[jasmussen]

I think @jameskoster may have some newer colors that should be applied across all sections? Not sure, but the color needs only a little bump to pass contrast. I.e. #858585 works. Don't go with that one for now, that's just an example, but I think there may be a higher level pass on these colors that can help.