Attachment #159015 [details] shows what Calc's rows/columns headers look like when you are running the Adwaita-dark theme variant (that you can toggle in gnome-tweak-tool's "Appearance" settings). As you can see, it has a light glossy "reflection" thingy that shines across half of the rows and columns in the headers, and not only is that reflection not functionaly useful, it is actually detrimental to readability as it is much more pronounced than when running a light theme (as seen in attachment #159013 [details]). I would recommend either for the gloss to go away entirely, or for it to be made much more subtle when running a dark GTK theme.
Nevermind, I was running version 6.3.x (I think), and it turns out that this issue got fixed in 6.4.x, from what I can tell with what's available in Fedora 32.
Thanks for retesting the issue with the latest version. Setting to RESOLVED WORKSFORME since the commit fixing this issue hasn't been identified.