I realize this is probably a question for Focusrite, but is there an easy way in Cubase to turn up or down the backing music when I am recording my vocalist (or bass player, or guitarist, etc…)? So far I’ve been just choosing everything in the mixer and turning it all up or down in unison but I end up having to remix everything when I do that.
Set up a group and route everything to it.
Depends on how you are recording, would need to know more on that to really give an answer. For me I monitor my singer through my mixer and set the monitoring levels for recording using the channel fader and the Aux master and send volumes (since we use reverb on the monitored vocals). With my mixer only the mic gain knob affects the level to my DAW, the fader is just for monitoring level. I typically raise/lower the vocals to balance the monitor mix then I simply adjust the output feeding our headphone amp to comfort. However, if I wanted to adjust the backing mix up or down instead like you mentioned I simply raise or lower the main output fader in Cubase.
Rev.
use the control room.
+1… tis what it’s there for 
exactly.
Use Control Room, set up a “Studio Bus” and use the studio sends and/or the control room functionality to maintain the studio bus for the singer.
Or, without the Control Room, setup a studio bus (group) yourself…