ThemeBuilder is a commercial plugin licensed via Atlassian through the Universal Plugin Manager (UPM).
After installation you will find that there are several plugins made available in the list of installed plugins, each of these plugins is capable of being licensed separately, however they do not all require a license.
If the ThemeBuilder Editor has a valid license, then the ThemeBuilder Engine will automatically be licensed too.
The Engine may show as Unlicensed if you have licensed through the Editor. As long as either Engine or Editor has been licensed, and the ThemeBuilder plugin is working normally, then this can be ignored.
To obtain a license you will first need an Atlassian ID, you can then either purchase a license or generate a free trial through the UPM, once it has been correctly configured, or you can purchase the license through the marketplace and then install the license by pasting the text downloaded from the licenses page in your My Atlassian account.