The colour of the rectangular cursor for selecting a box in my Calc page is black. How can I change the colour to make it more visible?
I am not really sure of your meaning.
I have an arrow shaped cursor in LO 7.2.3.2 but I can change it to a fat cross in Tools > Options > LibreOffice Calc > View and under Visual Aids > Cursor tick Themed. This is new to LO 7.2.
In Windows 10, I can change the colour of the system cursor in Start menu > Settings > Ease of Access > Mouse pointer. Under Text Cursor, I can change the colour and width of the vertical bar cursor.
But maybe you mean the selected cell border? I don’t think you can. If you have chosen a black background, the selected cell will have a black outline. The only thing you can really change is to show gridlines even if the background of the cell is coloured. In Tools > Options > LibreOffice Calc > View and under Visual Aids > Gridlines change to Coloured cells.
The cell focus rectangle (active cell cursor) colour currently depends on the application font colour. Starting with LibreOffice 7.3 it will be the application (OS) highlight colour, see tdf#142121.