Enable option for setting conditional rules on pivot views

Go to Settings -> Users & Companies -> Users -> under "Technical" section in your user, enable "Allow to Set Pivot Conditional Rules option".

Setup rules from the pivot view.

When you go to a pivot view with Default Rules, You will see the changes in the pivot according to the Default Rules.

Drag along the pivot cells to select the cells you want to apply the rules on.

Click on the Conditional Formatting button for setting the Rule, then select the Condition and enter the Value to base the condition on, choose the Background color and Text color and click the Save button to apply the rule.