Go to the Keynote app to add your style.
The new table style is added to the table styles at the top of the sidebar. You can drag styles to organise them however you like.
You can create a table style that matches the colours of a particular image. This can be helpful when you want to make a visual connection between the data in the table and the subject of the image.
The new style has a table title, a header row, a column and a footer row — all of which match colours from the image. When you apply the new style to a selected table, it imparts the colours to these features in the table (if the table has them).
Go to the Keynote app sidebar, click the Table tab.
At the top of the sidebar, Change table grid lines and colours in Keynote on Mac