The splitting line between two editors are imperceptible/invisible.

[zzeitt]

When I split editor windows horizontally or vertically, the splitting lines between windows are hard to be distinguished from background color.

Screenshots
current screenshot
where the horizontal splitting line should be
where the vertical splitting line should be

[gambolputty]

This can be changed with the sash.hoverBorder setting (docs). For example:

"workbench.colorCustomizations": {
  "sash.hoverBorder": "#285888"
}

[zzeitt]

Thanks for pointing me to the right direction, though what I want is to change the color of the border, without hovering.

This is how I finally got it to work,

 "workbench.colorCustomizations": {
        "editorGroup.border": "#24333f",
    }

And if you want to change other imperceptible borders, I have found the configurations:

 "workbench.colorCustomizations": {
        "activityBar.border": "#24333f",
        "sideBar.border": "#24333f",
        "editorGroup.border": "#24333f",
        "titleBar.border": "#24333f",
    }

Before	After

[zzeitt]

I just found that it has been stated in README (https://github.com/sdras/night-owl-vscode-theme#separate-the-editor-from-the-sidebar) 😂……