You will want to edit the CSS files contained in the relevant plugin, I believe.
Currently, as we slowly phase out support for the legacy Bootstrap 2 theme in AtoM, there are currently two Dominion theme plugins:
Bootstrap 2 will be fully removed from the next major AtoM release (version 2.9), so if you are making local changes or developing a theme, I strongly suggest you focus on the newer B5 theme.
In general, we recommend customizing the look and feel of your AtoM installation by creating your own custom theme plugin, which extends the base Dominion themes. This will make future upgrades easier, as all changes will be contained in your custom plugin, and it's much easier to move a custom plugin than to manually modify all the default BS5 theme files again after an upgrade.
We have some basic introductory guidance on how to prepare a custom theme plugin in our documentation, here:
The first section here also provides the necessary dependencies and instructions to recompile the Dominion B5 theme and/or your custom theme after making changes.
Good luck!