Theming changes between Totara Learn 9 & 10
Removal of Standard Totara Responsive and Bootstrap base
These themes were deprecated in Totara Learn 9 and removed in Totara Learn 10. This is predominately due to potential clashes between Bootstrap 2 (which these themes are built on) and 3 (which the new themes Roots and Basis, introduced in Totara Learn 9, are built on).
Core concepts
There have been no changes in the core concepts. Renderers, mustache templates, page layouts and CSS are still there and can be used to customise your theme. There has been considerable increase of the use of mustache templates.