reproduce:

• open a custom tree that does not use icons for its elements (e.g. process view)
• switch "File Icon Theme" between "none" and "Seti" and observe how the icon theme affects the indentation:
  

Note how children no longer align properly depending on whether they have itself children or not.

This broken indentation makes custom trees that have no icons basically unusable (if a file icon theme is used).

Since icon support for custom trees makes it first appearance in the January milestone, many custom tree implementations will not yet provide icons. These trees will be broken for users that use file icon themes.