Fix --fonts not working properly.

[Kodehawa]

This PR fixes #83 (--fonts "example example example" throwing unrecognized argument: example example). The resulting fonts get added to custom.styles as tested:

wal_steam itself has another issue, custom.styles doesn't seem to be properly used (it doesn't override fonts), but that's outside the scope of this PR.

The issue could be replicated both on my Arch machine and in a Fedora 32 VM.