If I know correctly, Plata theme is the default theme for Solus, so that it should be installed by default.
At this point i recommend you to check if you have any broken package by running on terminal:
sudo eopkg check | grep Broken | awk '{print $4}' | xargs sudo eopkg it --reinstall
All packages should be "OK" except udisks which is a know issue why it is showing as "Broken".
For curiousity you can run:
eopkg hs | grep plata-theme
to see what happened with it.