Add styling for nested tree view placeholders
Add styling for nested tree view placeholders

Nodes in nested tree views have slightly different look. This added styling
makes the respective placeholders look appropriately to maintain the illusion.

j0gurt/ggrc-core,hasanalom/ggrc-core,VinnieJohns/ggrc-core,hasanalom/ggrc-core,andrei-karalionak/ggrc-core,edofic/ggrc-core,selahssea/ggrc-core,NejcZupec/ggrc-core,kr41/ggrc-core,selahssea/ggrc-core,prasannav7/ggrc-core,selahssea/ggrc-core,prasannav7/ggrc-core,NejcZupec/ggrc-core,hyperNURb/ggrc-core,edofic/ggrc-core,plamut/ggrc-core,edofic/ggrc-core,hyperNURb/ggrc-core,josthkko/ggrc-core,plamut/ggrc-core,selahssea/ggrc-core,andrei-karalionak/ggrc-core,jmakov/ggrc-core,kr41/ggrc-core,VinnieJohns/ggrc-core,AleksNeStu/ggrc-core,AleksNeStu/ggrc-core,j0gurt/ggrc-core,andrei-karalionak/ggrc-core,jmakov/ggrc-core,jmakov/ggrc-core,jmakov/ggrc-core,AleksNeStu/ggrc-core,hyperNURb/ggrc-core,kr41/ggrc-core,NejcZupec/ggrc-core,hasanalom/ggrc-core,hyperNURb/ggrc-core,NejcZupec/ggrc-core,AleksNeStu/ggrc-core,josthkko/ggrc-core,hyperNURb/ggrc-core,kr41/ggrc-core,andrei-karalionak/ggrc-core,hasanalom/ggrc-core,plamut/ggrc-core,hasanalom/ggrc-core,VinnieJohns/ggrc-core,josthkko/ggrc-core,josthkko/ggrc-core,prasannav7/ggrc-core,edofic/ggrc-core,VinnieJohns/ggrc-core,plamut/ggrc-core,j0gurt/ggrc-core,prasannav7/ggrc-core,jmakov/ggrc-core,j0gurt/ggrc-core