The "Adding a theme" documentation tells you the following:

This will configure a default theme which can be enabled in the app/Resources/webspaces/.xml file by adding:
<theme>default</theme>

It does not specify the exact location (between localizations and default-templates).
Pasting the tag anywhere else in the xml results in Symfony throwing an exception:

XmlParsingException
[ERROR 1871] Element '{http://schemas.sulu.io/webspace/webspace}theme': This element is not expected.

So this can heavily confuse people.