FileSystemViewer toolbar colours inconsistent with rest of program.

The FileSystemViewer toolbar introduced in !262 includes some style input that is meant to mimic the existing MainProgram toolbar, but unfortunately I didn't manage to find a way to access the system-dependent colors Qt uses under the hood. This means that

1. Not all colours of the new toolbar will adapt to dark mode.
2. If the system colors change, the new toolbar doesnt change with them.