Your LimeSurvey version: 6.1.3+230612 
Own server or LimeSurvey hosting: own server
Survey theme/template: extends_fruity
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I have made an extended fruity them called extends_fruity. I am trying to make this as a default theme for all the new surveys. 
Under Configuration > Settings > Global > General, I changed "Default theme" to "extends_fruity". 
Now when I create a new survey the theme is not "extends_fruity", but "inherit[fruity]". 

Is there something else I need to do to change the default theme for all the new surveys? 
 

Default theme is used for the home page and 404 error.
For default theme for survey : check Global Survey