Step 1: Create a new blank presentation.
Step 2: Click on View-->Slide Master
Step 3: Format whatever images or fonts or backgrounds you wanna add.
Step 4: After adding or formatting your favorite stuffs, close the Slide Master View.
Step 5: Save the presentation as '.thmx' extension after the file name in the default themes available in your "C:/Program Files/Microsoft Office/Document Themes' folder
Step 6: Now close the presentation and open a new one.
Step 7: Now you can see your own theme in the 'Design' pane.
