ISAP/Library/PackageCache/com.unity.2d.tilemap@1.0.0/Editor/UI/TilePaletteOverlayLight.uss

.unity-tilepalette-brushinspectorpopup
{
    border-left-color: rgb(130, 130, 130);
    border-right-color: rgb(130, 130, 130);
    border-top-color: rgb(130, 130, 130);
    border-bottom-color: rgb(130, 130, 130);
}