Please help us help you and fill where relevant:
LimeSurvey version: LimeSurvey Community Edition Version 6.16.11+260303 
Own server or LimeSurvey Cloud: own server
Survey theme/template: fruity
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I upgraded our LS server from version 3 to 6. Everything is running fine, except that theme options are not available anymore in the survey settings. We use about 20 themes, all inheriting from the Fruity core theme. I added the following line to all the subthemes in order to have them working on LS6 (<compatibility>        <version>3</version><version>6</version></compatibility>). But when we edit a survey using one of these 20 themes, in the panel "Theme options" we cannot see any field. If I add the following line in the theme config.xml file (<engine><optionspage>core</optionspage></engine>) I only see the toggle button "Inherit everything" but not the option fields. I thought I would need to copy the options.twig file from the core Fruity theme, but it is empty. What am I supposed to do?
Thanks you for any suggestion.
Pierre