Translations:New Template System in LS3.x/113/en
From LimeSurvey Manual
They will use the inheritance system. It means that if you add a CSS/JS file to a theme, but it's not present in the theme, then the engine will look into all the mother themes of this theme and will use the first one it finds. So if a user extends your theme via the theme editor, all those files will be inherited in his theme. The keywords refer to this inheritance system.