Hi @Jasper Blikman, Please check the default theme of your module and the UI flow; if the theme selected in either of them is being consumed from another module, then you can't edit it from the consumer module; you will have to edit it from the producer module, or else you can create a new theme in your module, set the base theme to the one you already have, and then make the new theme your default theme, and then you can override the CSS using the new theme.

Thanks
Gitansh Anand

Hi Jasper,

Please reopen the Service Studio, or raise a ticket with OutSystems to seek help if this does not resolve the issue.

Thanks 

Add back the default OutSystemsUI Theme as dependency

Link your stylesheet to the OutSystemsUI Theme

This will make the Theme Editor button be enabled again

Open the Theme Editor, click 'Reset all styles'

Link your stylesheet to your original theme (if you had one)

Remove the default OutSystemsUI Theme

Publish, done

(the issue is: the Theme Editor button being enabled is dependent on multiple things, one of which is... the OutSystemsUI Theme needs to be used/linked to the page/webblock/module)

@Jeroen Bindels solution worked :)

Thanks