I'm setting up M2 and what a pain it is!!! I'm trying to install 2 really popular themes (Ultimo and Porto) basically I got so frustrated with the issues of the themes and the display I deleted the "Default store and Website" from the tables 'store', 'store_group' and 'store_website'
Stupid I know!!.... no i do not know how to get a working site back
Please help
It sounds like this is not a production and you are playing with themes, settings etc. I would suggest you to reinstall the Magento or having the hosting to check if they have recent available backups of your DB
Agreed with @kostritsaalex recommendation. You may also need to follow the installation instruction provided by the theme providers. Those steps may help you install the them right way. You can find the documentation for Porto theme below:
there must be a way to reinstate the default theme into the DB