First go too Tool>Customize>Icons> filter the thing you want to modify.>select it>Copy text. Then go to your user folder In Windows: C:\Users\YOURNAME\AppData\Roaming\FreeCAD\Gui\IconSets
Make a file MyIcons.txt also add a directory (If needed,since some stuff will happen automatically if you press save in the icon window.) called MyIcons Open the TXT file and add the lines (or paste your own that you copied above):
Then add following SVG to the directory: You can also get the original and modify it by pressing the export button in the customize>Icon window.PartDesign_Body, iconset:MyIcons/PartDesign_Body.svg
PartDesign_Body_Tree.svg, iconset:MyIcons/PartDesign_Body.svg
Restart Freecad
This is how mine looks; I will probably release my icon pack in the future when it is done.