as the space between the icons\objects depends on the icon size and not every skinner set this parameter first I suggest to try different themes as every skinner use different settings and in the end you will find the one fitting your needs
anyway opening the .ini file you can always tweak the dockbackground, actually if you don't find any [Tile] configuration you have to add the following lines:
[Tile]
Image = Docktile.png
ResizeIconByPercent = 95;
OffsetIconByPercentX = 50;
OffsetIconByPercentY = 50;
moreover you need to add in the same folder of the .ini file a transparent Image .png named (in this case) Docktile.png,
To have more space just decrease the size to 90\85\...
btw embedded in my "WhiteCristmas Docks" you can find a transparent .png ready to be copied and used
https://www.wincustomize.com/explore/objectdock_backgrounds/289/
let me know ! 
