This is a feature request rather than a bug, but it is a bit of a usability problem. When using a dark theme, the cell background is the dark colour from the theme. The font is light to compensate so text is readable. However, if you open someone else's document where they've set up borders and text specifying black for text and/or borders everything is illegible because they're expecting it to be opened in a standard office program which uses white cells. One workaround for this is changing the theme in the application. However, this is a workaround rather than a fix. It would be better to add an extra option in Settings > Configure Calligra Sheets > Colours which allowed the user to override the cell background colour. Reproducible: Always
Found the same behavior on Sheets 3.1.0 after opening a document while using a dark system theme even Breeze dark the cells will take the system colors along dark or black text making difficult to see and edit the documents. I agree with the original user on this topic. The cells need to follow the white and black borders even with dark color themes.
Recently retested on the last alpha 3.1.89 on unstable branches by switching color schemes and the results are pretty good the workspace stays as white with black fonts even if the rest of the application is following the system color scheme. Since the original report talks about the dark cells the only noticeable detail is that some cases a darker theme can make the cell cursor difficult to see against the current cell border. Not really sure if that enters on this report.
I think this might be a duplicate of https://bugs.kde.org/show_bug.cgi?id=234173, in which case it is fixed as of 3.2.1. Nowadays the application follows the dark theme, while the cells stay black text on white background.