For that topic to be searched is a need to index these. Each theme can be indexed / re-index by right-clicking on the the topic list and select “Index theme now”, as shown below:
For that indexering to work, the following properties on the topic be filled out (highlighted in red below)
If properties mention above change the theme must re-indexed
It must also re-indexed if the content of the theme is changed, for example, when the data set has new objects.