How to change the drop marker color in CSS ?

[haoadoreorange]

As the title, I can't find how to change the drop marker (the line under of above the tab drop destination) even after inspecting :(

Please help :(

[piroor]

There is the hint:

treestyletab/webextensions/sidebar/styles/drag-and-drop.css

Line 10 in 7b9e52e

--tab-dropmarker: -moz-fieldtext;

For example:

:root {
  --tab-dropmarker: red;
}

[haoadoreorange]

omg thank you so much, I'm an idiot.

[haoadoreorange]

but just out of curiosity what the actual element that was doing it ? I couldn't find it for some reason, no changes I made to outline or border worked at all.