To change the theme of an existing competition, follow the below steps.
Select Assets in the left hand side dashboard
Select Competitions
Select the competition that you wish to change the theme of
At the next window, select Theme
Then click the red button that displays Change Theme
Hover your mouse over the theme that you like the look of and select Preview
Here you can select a different colour combination that most appeals to you and your production
When happy hit Use This Theme
Follow the on-screen prompts to confirm your selection, and voila! You have successfully changed the theme for your competition. Congratulations!