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

Themes are significantly different in LS6 than in LS3. I don't think you'll be able to use your LS3 themes in LS6 without problems.

<optionspage>core</optionspage> means: do not use the option.twig file. In you case that is correct, since options.twig is empty (as you say).

Are there any options described in the config.xml file in your theme?

You may indeed have to do some adjustments, the way themes work in version 6 is different from version 3. I rewrote my base theme, this was quite some work.

Tammo