N00B question: How can I control the Spotify app volume?

I stream Spotify to my speakers via a Chromecast. I would like to be able to adjust the volume even when Spotify is not the active app. But when I try, my touchbar slider (or trackpad 3-finger swipe set to "volume up") only affects the system volume.

Is there some way to tell it to adjust the Spotify volume?

Thank you in advance!!