How to I change separator spacing in any theme ? I would like to know, because for me it seems spacing is too long between the icons.

Try: Preferences --> Appearance --> Icon Spacing.

That aslo affects distance between icons. I only want to change distance between separator and icons only, NOT between icons. Even when I do change icon spacing to minimum, then it's still not enough for me. Maybe I could edit some file manually which affets dock's separator spacing.

now i don't know if this is something you would like to do but i remove the separators. i think with most themes it gives workshelf a cleaner look.

Separators always take the same space as a normal icon. There is no way to change that without also changing the normal distance between icons, sorry...