Hello Niklas,
please make sure to create your theme with the UI5 version that matches the UI5 version you’re using in your application. Else there could be some incompatibilities as observed by you. Also it’s the safest option to use the standard theme of the UI5 version you’re using but be aware that this has changed over the course of time, e.g. 1.60 uses the Belize theme as standard while nowadays the Quartz Light theme is usually applied.
In your screenshot it shows “based on sap_fiori_3”. Is there an option to select Quarz Light instead if you’re working with a current UI5 version? Please keep in mind though that all standard widgets provided by Simplifier were generated on the basis of UI5 version 1.60 so if you’re using them this might also cause some conflicts if you apply a theme created on a higher version than this.
Regards,
Armin