I have the bitmaps for NX6 but have been wanting to get the 7.5 icons as well.
The way I find the icons I want is to use the Explorer search to search for key words in the icon directory. A good viewer like xnview (which quickly displays thumbnails) is useful too.
If you are checking that specified geometry lies on given ranges of layers the key is to set this attribute:
# "entity_type1, [entity_type2...] | layer_range"
(list) entity_layer_map: {
"SOLID_BODY, SOLID, SHEET_BODY | 1-50",
"_CURVE | 51-100",
"SKETCH, LINE, ARC, CONIC, SPLINE, DATUMS...