Reinstale seguindo o comando:
sudo apt-get install --reinstall light-themes
Quando pesquisado por apt-cache search ambiance :
light-themes - Light Themes (Ambiance and Radiance)
ubuntu-ui-toolkit-theme - Qt Components for Ubuntu - Ubuntu Theme
focuswriter - Fullscreen distraction-free writing program
themecolorizer - Application for recoloring themes
Isso ajudará a instalar / remover pacotes relacionados a temas.