Hi @PSewall,
the theme files are located in your theme folder which can be found in the Design-Tools(Marketing → Files&Templates → Design-Tools or Design Manager ).
If you don’t see this option contact an Admin from your Hub and request the rights for access to it.
Once you’re in the Design-Tools open the theme - according to the source code it should be inside the “intrinsic-digital” folder. Somewhere in this folder you should find a module folder with a module called navigation, header navigation or similar. Open it and you’ll find the CSS part you’re looking for in the module.css area.
Important: It seems that it’s is a custom theme set up by an agency - inform them that you’ve made some changes. Otherwise they might overwrite it if they update your theme
best,
Anton