We had covered earlier with the steps required for changing the default theme in Keynote. But if you want to change your theme after preparing the Keynote presentation then you can use the File -> Choose Theme option.
Step 1: Launch Keynote app on Mac and click the File menu.
Step 2: Select Choose theme from the menu list.
Step 3: In the Choose a theme for your presentation, select the theme of your choice.
Step 4: Select appropriate values for Apply Theme to option i.e whether you want to apply the new theme to All Slides or only selected slides. Similarly specify the Slide Size i the Slide Size drop down.
Step 5: After making all required changes click Choose button to apply the theme.