Opacity for colours set within vim

I use a transparent terminal background on URxvt, which I set within my Xresources file:

URxvt.background: #2f2f2f

In vim I have line numbers on the left hand side with a different background colour, as set in my vimrc:

highlight LineNr ctermbg=8 ctermfg=7
highlight CursorLineNr ctermbg=8 ctermfg=4

Is there any way to set a transparency for ctermbg in vim, such that the background of the line numbers is transparent? (I am aware that if I set the transparency for my terminal through my compton config, instead of setting the background transparency in Xresources, then I could achieve this result. However I have tried this and I am not happy with the way it distorts the colour of my text.)