To use the search feature for that theme you need to create a content key from your admin panel. Generate a new Content API Key by using the following steps :
1. From Ghost Admin Dashboard click on Integrations
2. And then click add custom integration
3. Write any name for that custom integration and then click Create
4. Now the key will be generated, just hover over it and copy the code. ( Content API key )
Congrats! You got your ghost content api key for your blog. Now use that in the next step.