@Henri, you did not answer my earlier question as to whether you were updating from a version prior to November 2016. Based on your responses, however, I believe that was the case. If so, let me explain what happened:
- Manager originally supported only very minor changes to sales invoices through a Settings category called Sales Invoice Template.
- Then were introduced what were called View Templates. These were written in HTML and also affected only sales invoices. They allowed much greater freedom to customize sales invoices.
- In November 2016, at version 16.11.24, Themes were introduced. Themes were written in Liquid and applied to all forms. You could have as many different themes as you wanted. For the sake of legacy users, the program still supported existing, obsolete View Templates. But you could not create new ones.
- Approximately 8 June 2017, at version 17.6.0, themes were replaced by Templates. These could be copied from a web site, edited, or written from scratch, also in Liquid. At this point, support for the obsolete View Templates was discontinued. However, existing custom themes were automatically converted to custom Templates. So there was a seven-month period during which you could have brought your custom presentation up to date.
- In the two weeks since they were introduced, the Templates feature has already undergone substantial improvement. First, multiple templates were incorporated. Then the ability to designate a primary and secondary templates was added. All this was done in response to feedback from users trying out the initial implementation.
The future does not lie with reverting to a scheme two generations old. There will be no support for older versions of the program. And Guides are written for the current version. Questions on the forum are answered from the perspective of current or very recent versions. So you should focus on choosing or writing a template that meets your need.
Templates are edited directly within Manager. After you have created a template according to instructions here: [17.6.0] New workflow for custom templates, under Settings => Templates, click Edit for the template you want to edit. You will see the code. Edit the code within the viewing window and click Update.