This might be more of an MS Office issue than it is Canary where Excel doesn't use the write label text color to play nicely with the Dark Gray office theme but I noticed it looked like this (white label text). It was fine when I changed to a white theme and also interesting is that when I did that and then changed back to dark grey, the label text was black and readable using the dark grey theme. When I closed and re-opened Excel though after that change, it went back to white text.