nix: add theme setting to nix config

[mBuschauer]

Added theme option to nix config, still can't figure out how to fix this line:

Environment =
          lib.optional (cfg.adminUrl != null)
          ["TAILRAY_ADMIN_URL=${cfg.adminUrl}"]
          ++ [
            "TAILRAY_THEME=${cfg.theme}"
          ];

[NotAShelf]

Sorry for the delay, and thank you for your patience :)