how can i hide the separators in the bookmarks tab

Pathduck

I recommend you learn basic inspecting the UI so you can figure stuff out in a couple minutes instead of waiting for hours for a solution from someone else.
https://forum.vivaldi.net/topic/16684/inspecting-the-vivaldi-ui-with-devtools

So you're using separators to "center" the bookmarks? There are better ways to do this but ok:

.bookmark-bar button .separator {
    border: none;
}

A much better way would be to use this instead of separators:

.bookmark-bar .observer {
    justify-content: center;
}