By default LibreOffice Calc will have white cells with black fonts after installation.
Extremely frustrated by the default automatic fonts…
And how do you do that?
If you want to change the font or background color you can do it via direct formatting or via the cell styles in the sidebar (Ctrl+F5).
If cell colors have to be changed often, formatting with cell styles is recommended in any case and don’t use direct formatting.
Open an empty spreadsheet and create all cell styles you need.
Then save the spreadsheet as the document template “Default”.
When you open Calc further, you will get your template and can work with it.
There are more possibility to change the colors with the so-called “Conditional Formatting”. I assume that this is not meant.
It is also conceivable that you have made a change in the Tools>Options>LibreOffice>Application Colors
menu.
There scroll to the right at “Custom Colors” to the heading “Table Document” and below at “Text” set the color back to “Automatic” or “Black”.
See also: English documentation