Classic white background cells in blank format

You mean you don't want the gray background in the headers? This is not Excel so you might not actually want to do that but here are two options:

Be just like Excel and don't use header columns and header rows. Delete them. This might become a problem when you make charts, though. Charts use the headers. You can override the defaults each time but charts will be a lot harder to make.

Or, select the cells and change their background color to white. You can also change all cell borders to 0.35pt if you want it to be completely plain. However, if you make the headers look just like the regular data cells, you may forget that they are special. For one thing, a range address like "B" (instead of B1:B20) to address the entire column does not include header and footer rows.

If you do change the color to white (and the borders all to 0.35pt, if that is your choice), you can add your new style as a table style. If you move that style to be the one at the top left (the first one on the list), it will be the default for new tables. Save your blank document with your new table style as a template so you can use it for future projects.

You could take a look at the sixth table in the Table Styles (down - right).

Header rows/columns and Footer rows show cells with a blank background.

You might turn off the "Alternating Row Colour". Hence all cells in the body rows will have a blank background.

Paul.

Look at the picture in your first post at the bottom right.

Right there you can turn on/off the "Alternating Row Color".

Paul.