Separator-size location

[koumy]

Hi all

I have pbs to find this location , have another item horizontal like that

Thanks for help 

[3am]

What do you want to know?

The inside is just Fonts & System Metrics > BTNFACE

The borders are Lists, Menus, & Tabs > ListView > BORDERCOLOR and Explorer & Shell > Explorer > TreeView > BORDERCOLOR

[koumy]

Oh I wanted to know how to get rid of gap at some locations

Thanks for help , it solved regedit but not some third app like everything search but I wonder if it depends on visual style for it as it didn't remove the "separator" , at least I 'd like to make the zone thinner
still there after :