What is this small bar called in a group?

[Rain-Gayming]

I'm trying to make it transparent to fit the rest of my nvim set up and i assume its from nvim-tree, current groups i have are:
NvimTreeNormal
NvimTreeNormalNC
NvimTreeNormalFloat,
NvimTreeEndOfBuffer
NvimTreeWinSeparator,

thanks for any help

(the bar to the right of the tree)

[alex-courtis]

:NvimTreeHiTest may be of assistance to show you the actual value and appearance of each group.

The vertical bar running down the entire screenshot is NvimTreeWinSeparator

:hi NvimTreeWinSeparator guifg=blue guibg=red worked for me. Check :NvimTreeHiTest to see what's going on.