I see that the theming arrangements for 3.0 are different and I will need to compose new html template and css files on the new basis. Is there a guide for this similar to the existing one for 2.5 please? I'm having trouble following the information in the upgrade guide, it isn't clear where I get the "html template" from, for example.
The only difference that you need to worry about with a Mendix 3.0 theme are documented in the release notes that you can find here.
Also from the notes:
"For 2.5 to 3.0 theme conversion, it is recommended to create a new index template based on a 3.0 index template rather than modifying the old index template. As an alternative, follow the steps below."