diff --git a/gnu/system.scm b/gnu/system.scm
index 8e05254..a8744ea 100644
--- a/gnu/system.scm
+++ b/gnu/system.scm
@@ -421,6 +421,9 @@ export INFOPATH=$HOME/.guix-profile/share/info:/run/current-system/profile/share
 export XDG_DATA_DIRS=$HOME/.guix-profile/share:/run/current-system/profile/share
 export XDG_CONFIG_DIRS=$HOME/.guix-profile/etc/xdg:/run/current-system/profile/etc/xdg
 
+# 'GTK_DATA_PREFIX' must name one directory where themes are searched for.
+export GTK_DATA_PREFIX=/run/current-system/profile
+
 # Ignore the default value of 'PATH'.
 unset PATH