Install x11-themes/mate-icon-theme which is needed when this package is used in the MATE desktop environment.

Packages describing “mate” as local USE flag

Package “mate” Flag Description
mate-base/caja Install x11-themes/mate-icon-theme which is needed when this package is used in the MATE desktop environment.
x11-themes/vertex-theme Install Metacity theme for mate-base/mate
x11-themes/zuki-themes Install Metacity theme for mate-base/mate
x11-themes/arc-theme Install Metacity theme for mate-base/mate

All packages providing a “mate” USE flag (3)