Toolset has an option for loading Bootstrap 4. Where does the source CSS file live? If I customize it, is there any risk it gets overwritten when a new release of Toolset comes out?
No, it is not recommended to hack Toolset plugin files, any change will be overridden when you upgrade Toolset plugins.
So you can disable Toolset options for loading Bootstrap 4, then load your own Bootstrap 4 files manually
This is the technical support forum for Toolset - a suite of plugins for developing WordPress sites without writing PHP.
Everyone can read this forum, but only Toolset clients can post in it. Toolset support works 6 days per week, 19 hours per day.