Themes and Totara
Base
Base theme is almost a completely clean canvas containing only font icon CSS (no module CSS) and a very basic layout. It is also injected if it is not included in the Theme change as otherwise font icons will not work. Any Theme built on this will need to include all CSS and possibly overwrite a number of mustache templates and renderers (as these have been built with Bootstrap 3 in mind).
Legacy
Legacy is a basic Theme that includes styling for all non-Tui core functionality. It has minimal colours and includes Bootstrap 3 CSS along with SCSS files for all components. All core functionality is designed to work with this Theme.
For Totara 13-18, this is the best theme to base custom Themes off.
Ventura (Totara 13+)
Ventura theme is built off Legacy with a small number of colour tweaks. It also includes some settings to allow colour changes which may be enough for some instances.
Inspire (Totara 19+)
Like Ventura, Inspire is built off Legacy, but includes a number of UI improvements such as a new primary navigation.
For Totara 19+, this is the best Theme to base custom Themes off.
Deprecated/removed Themes
From time to time we will be deprecating and removing Themes as technologies change and improve. It is possible to continue to use these Themes after they have been removed by keeping the files in the Theme directory, however they will be unsupported and are highly likely to have issues.
Roots / Basis
Roots and basis were deprecated in Totara 14, and removed in Totara 19. These Themes were built with Less, unlike our current set of Themes which are built with SCSS. These Themes do not style all of Totara correctly in recent versions, so it is strongly recommended to use one of the Themes mentioned above instead.