Why the hightlight color is wierd?

[MKrbm]

I'm digging around the highlight setting to change nvimtree color.
But found no highlight group changes the color below the files as you can see in the picture.
How to make entire nvimtree pane red (for example) ?

[alex-courtis]

See :help nvim-tree-highlight

I think you might be after NvimTreeNormal e.g.
:hi NvimTreeNormal guibg=red

[MKrbm]

I already set :hi NvimTreeNormal guibg=red, which is done in the setting on the right side of ss.
However, the color doesn't apply entirely on the left window.

[alex-courtis]

You can see all the nvim highlights via :NvimTreeHiTest

There are three highlight groups that affect that area: NvimTreeEndOfBuffer, NonText and EndOfBuffer.

NvimTreeEndOfBuffer links to EndOfBuffer by defaults. nvim links EndOfBuffer to NonText by default.