Did some more tests with the J15 site and a local copy.
I have re-installed everything with CB Packager. Seemed to solve most display issues i had before.
Still one thing I am curious about:
The mode "Viewer" is nice to test a custom template, that i am working on. Users don´t notice any changes, and I can test the new template. How does it work? e.g. In the source I can still see:
<div class="cb_template_default">...
Usually, this Class changes to the selected template name. Should it change in mode "Viewer", too?
If the admin is testing with "BootStrap", shouldn´t it change the source output? Do you just override/swap the whole CSS file's location?
That would explain some of my mixed results. Because i make use of that class_with_template name.