Theme CSS libraries always loaded after module libraries
The proper way to modify the loading order is weight property as described here.  The lower the weight the further up your library should go.

Try something like the following in your trent/trent.libraries.yml

bootstrap:
  css:
    base:
      libraries/bootstrap/bootstrap.min.css: {weight: -50}


You might have to adjust the number to get it to desired position.

NOTE: weight is in the process of being replaced by dependencies. Work done can be seen in this issue and when ready this sort of problem will be easier to tackle.