symfony/symfony-docs — comment on commit 78fbe0505f42b091eca4dd42b780291e3eed950d
File: cookbook/form/create_custom_field_type.rst line 110
By stof:

with the use of traits, you need to know the other form theme used, otherwise you will either always import the block of the core theme again, thus overwriting changes done by the previous theme or have to use the previous theme each time.